I've had quite a lot of experience on eBay: my wife has been selling antiques there full-time for 2+ years, and I help her out, as well as sell musical instruments as a sideline. I have also bought and sold a fair amount of scuba gear.
Here's a few things that I've learned:
Allways check a seller's feedback. Most sellers are everyday, honest people using eBay like a classified ad, however you find everything from large retailers looking for another outlet to the scum of the earth. If in doubt, email former customers listed on the seller's feedback page.
Make sure you understand the terms of the sale. Can you return it? How much is shipping?
Check the price against what you would pay at a regular internet dealer. People often bid an item up past it's real value.
Use your " My eBay" page to bookmark multiple auctions of the same or similar items. This has often helped me to find the best price.
If you buy something that just dosn't work out, like a wetsuit that dosn't quite fit, you can allways resell it.
My best bargains: Mares Ruby reg w/octo and an older air-integrated computer for $225, new BARE 7mm wetsuit for $160.