I have sold a few scuba items on ebay but the few cheaper items I bought on ebay didn't work out. Regarding musical items..guitars and guitar foot pedals...I've had good luck buying and selling.
For me the key to selling on ebay is to take good pictures, write good copy including the reason you are selling and setting a price you can live with as the minimum and letting it go from there. In some cases I've increased the minimum price on an item that didn't sell previously and this time it sold!
As a buyer, look at the rating...percent as well as number of transactions,good descriptions and pictures and ask questions if necessary.
Ultimately, you have to know how much something should sell for. I always look to see what similar items went for on ebay in the past.
The best items whether you are a buyer or a seller are things that are no longer made. If someone really wants it you can set a higher initial price. If you really want it you can justify paying more. If it's something currently available it's hard to do that.
Lastly, there are some items that are just popular. Stick to those and if you don't like your purchase you will have an easier time selling it as long as you bought it at the right price in the first place.
I haven't had much luck with scuba gear and therefore quit trying but I know others have (as mentioned in the posts above).
What do they say...ebay is your friend.