Theres lots of things you should be looking at - and luckily with e-commerce sites, theres a lot of ways to start marketing your business for free.
As this is in process, it's never too early to start, and if you have a site name, you can start doing some pre-launch marketing and promotions via blogs/forums and social networks - also using sites like reddit and digg you can create a bit of buzz about your service or usp and hopefully get people excitied. Make the most of the time you have, and if you plan to start selling before christmas - as much work as possible now, will help much much more than local newspaper advertising.
In my experience, e-commerce sites perform better with online rather than offline advertising - but this depends on the message. If it's an INCREDIBLE special offer - like "All games free today!" then it does'nt matter where the ad is placed, you're going to get activity - but the pre-launch work I was talking about before, is like the ground work you are putting in for getting your important first (and repeat) customers.
Your efforts can bring great rewards, and all it costs is your time. Read up on some blogs, and seo practices, join gaming forums/social networks - then start writing about your fantastic new venture!
All the best - I hope it is successful for you!