eBay is the world's most popular on-line marketplace. From a BDSM perspective it is a very good source of cheap SM equipment and pervertables. There are also a large number of retailers selling fetish clothing.
As with any purchase it is always worth checking out prices from a number of different sources before committing to buy something. However, eBay is often considerably cheaper than most stores (even on-line stores). For instance, Vetrap is typically about £2.50 a roll in the UK and on eBay it can be bought for as little as £1.30 a roll.
A variety of key words entered into the eBay search engine can bring up a wide range of products of interest to the cost conscious pervert!