There are thousands, indeed likey millions, of discussion forums available online. These are places where people interact posting their thoughts, comments, and opinions, in threaded "discussion" areas on a website. Some of these sites are topical and posting rules are strictly enforced. Some allow you to post links to your commercial website and some don't. Some allow open discussion about your business, some restrict business discussion to relevant threads or selected forums and some don't allow any commercial discussion at all. It is worth your time and effort to seek out those forums where you can post links to your selling site(s) and/or discuss your business.
Using Google to search for: +"your product" +forum, or +"your business" +"discussion board", will get you more results than you can look at. For example, searching for +antiques +forum provides nearly twelve milling returns.
As you find those forums that you will want to participate in you will want to group them into two categories: general forums, and topical forums focused on the product(s) or service(s) you offer. Bookmark (save as "favorites") the URL (address) to these sites. Saving them in separate, appropriately named, folders makes further use much easier. Gall these folders "general" and "topical" or "focused" or some other suitable names.
Your next step is to join those forums you wish to participate in. Select forum that let you list your website(s) and/or selling site(s) and other business information in your profile and that also allow you to have a signature that contains at least one link to your primary website.
If the site does not allow commercial linking but seems to contain a large number of members you would like to reach AND they do allow linking to general information websites (this is most often the case on restricted sites) then you need to create a web page (or have one created for you) that contains extensive relevant information on the topic of reference at the forum. A Frequently Asked Questions (FAQ) format is often suitable for this. Make the site "sponsored by" your main selling site with a banner and link back there. This is a powerful, indirect, and acceptable, method to get exposure for your business in places where you cannot blatently advertise.
Be sure, however, that you carefully read the rules and regulations posted at each site and follow them completely. If a site is simply too restrictive for your purposes find another. There is no lack of available sites. Don't spam the site with off-topic posts. Get truly involved in threads that are on topics which are both of interest to you and, in some way, related to your business. Become the resident expert on one area of information so people eventually seek out your advice and information.
Ask questions. Show genuine interest in topics that are the focus of the site. Ask for explanations, suggestions, "how to" solutions to problems you are aware of. If nothing else the information garnered can form the basis for your FAQ page(s). When appropriate offer a link to your information site. Provide free help and information when you can. If suitable use something on your own website as an example or to make a point. Ask for someone to check out your site or some particular aspect of it. Ask for a review of your blog post(s). You do maintain a blog, right? As long as you have reasonably thick skin (you will get "flamed" from time to time) critical assessment is a powerful thing. It tells you what you may be doing wrong and it tells you a lot about the person offering it. Again, as you become more and more known, people will seek out your council in return. These people are always excellent prospects for your business.
Read and "listen" to what others are saying. People on these forums are most often asking for help in one form or another. Offer solutions. If someone posts about their blog be sure to visit the site and post a comment or two. Ask if you can provide a guest post for their blog in exchange for attribution and a link. It's a great way to get exposure while providing real assistence to someone else. All but the most prolific bloggers would be foolish not to accept suitable guest posts when offered!
One of the most important things you can do, once you have established yourself on a relevant forum, is to ask the members what THEY are looking for in the kinds of product(s) or service(s) you offer. Ask them what they want. And listen to their answers. It's a great way to find out if you are offering the right product/service mix. Inquire about price points, what they are willing to pay, what constitutes too much, what price is so low it makes them suspicious as to the quality or validity of the offer.
People like to share their opinions and will, usually, tell you honestly what they think. Periodic checking on a variety of sites lets you gather exceptional feedback as well as exposing your product(s) and/or service(s) indirectly and acceptably in most forums.
Broader questions like "What is the most important thing a selling site can offer?" or "What information do you look for about a product/service before making an online purchase?" can lead to great discussion. Take part in the various threads that develop from your opening questions and find ways to appropriately insert your own information into the process. It's always fair to ask someone if they are prepared to pay for a solution to their stated problem and even how much they might expect to pay. This can easily lead to a direct contact and a sale.
Develop FAQ pages on topics that come up in this fashion and use those pages as references on other forums. Let the FAQ pages be the free information you offer and they will drive traffic to your business or selling site. Create and give away free web-based courses. These courses can contain ads and links to your commercial sites. Everyone likes to get free stuff. Most people will at least look at such an item. And many will follow the link(s) to your selling pages. Truly giving away something of value is one of the most powerful ways to build a very successful business.
Be sure to have a list of resources and/or websites that people may want. Include, among them, links to your own sites. Don't foget to provide links to your "sponsored" FAQ pages. The more information -- good information! -- you offer the more likely your page is going to be bookmarked and re-visted over and over again. Long term repeat visitors are almost certain to follow your links to your e-commerce sites.
By becoming the resident expert and problem solver you develop trust and a following that will be more than willing to do business with you when their need arises. When that happens you want to be there for them. You want your sites to be easy to find. Hopefully they will have been bookmarked and you will be regularly sending emails and/or newsletters to these people over the weeks and months.
Over that last few years I have sold my products and services to hundreds of people who have come to know me in one single forum that I frequent. They've come to trust me, to follow the links I provide, and, when they need something I offer, they know where to find me. I repeat this process in a number of forums and my business is good as a result. Don't be afraid to promote yourself. Just find the right ways to do it!