Be a participant, not just an advertiser. Get involved in the discussions so that people know who you are. And, never, EVER, send junk mail. It is NO way for legitimate business to advertise, and its the fastest way to destroy your reputation. First off, guarantee what you sell.
Many people are very wary about sending money to an unknown entity.
Have some very low priced items to help get first time net-buyers involved without them taking on what they perceive to be a big financial risk. If you are not adept at designing professional looking web pages, have someone else do it for you. A poorly designed or amateurish looking web site conveys the impression that you don't care about your business, or are a poorly run operation. If you have a web site that looks like a 9 year old designed it, nobody is going to take you seriously. This shows that you mean business, that you are here to stay.