Oh jeez is that a loaded question. There are so many ways to promote it and get more sales. Focus mainly (right now) on promotion and getting visitors. First and foremost you need quality content that your users want to see, easy to navigate, etc. etc. You should get inbound links for your site with the Anchor Text of the keywords you are trying to rank for -- this will help you get higher in the Search Engine Results. There are a million things you could do to promote your site, from Blog / Forum Commenting, Newsletter Creation, Contests, and of course buying advertisements (adwords, banners, etc). I suggest, if you can afford it, is to track down some website owners who are in the same general niche as you, for instance a Baseball Forum, tell the site owner you are willing to donate a one (or more) caps for a contest they hold. Website owners love free swag and you'll get a shout out and mention on their site. There are so many things you can do to promote your site, and most of them you can do for little or no money! Good luck! Edit: If you haven't yet already, sign up for Google Analytics and Webmaster Tools, submit and verify on both programs and create a Google Product Feed (Froogle Feed) this will help get your products out there on the web
Building backlink with keywords as your anchor text. Build it consistently, you will see some great organic traffic after 1-2 months. Then continue from there. remember, quality content is the key.
sport forum would be great. for long term, you can create own sport blog etc. and alternatively you can use auction site, there are where buyer met.
Yes, you may create your own blog or forum. You may also market them to socila networking sites like facebook.