People love deals. That is why you can make use of coupons to entice people to buy from you. This can happen anywhere you sell online.
Suppose you sell on eBay. Then, you can offer a bonus coupon for subsequent purchase. You can even add customers that buy from you onto your mailing list and alert them any time you have new coupons available. Just be sure that you use double opt in (meaning they have to confirm) and always give them the option to opt out. Make that easy for them otherwise they'll accuse you of spamming them.
To implement coupons is fairly straight forward. You simply setup a form on your sales page and if they enter the coupon correctly, you redirect them to another sales page that has the discounts preset.
Hope this helps. Get more helpful advice on selling online (especially auctions) at Auction Startup