you can send customers and prospects periodic updates about new products and store promotions or you can use google adword
Actually just read/listened to audio about this as I am interested in getting into eBay selling too... Michael Senoff's Hard to find Seminars website has a lot of information on how to get started and market on eBay (and it was free, so even better). I found it really interesting and helpful! Google Michael Senoff + hardtofindseminars and then look under the audio's on Selling on eBay. Hope this helps!