I don't know of anyone having problems on Aquabid but as OkieMavis said, I always check reputations before buying anything. The same goes for E-bay when it's me on this end. I also get most of my supplies and some fish on line and find that the typical vendor is trying very hard to keep their good reputation. What that means is you buy 6 of something and 7 arrive or there is a free sample of a related product or some other measure that makes it a memorably good transaction.
The worst case that I can remember was when I bought something, paid for it and never heard from the seller again. About the time that I started to worry about it, the package arrived at my door. The negative in this for me was that I was not used to silence after the sale since most vendors make darned sure that you know your product is on the way and also make sure that you know the moment it gets to the shipper.
In a recent transaction, I asked a vendor if they had a product to recommend that I did not see in their on line catalog. When I didn't hear from them right away, I ordered a similar product from their catalog. I got a phone call a few hours later asking if I really had wanted to order the product that I had been asking about or if I wanted the specific item from the catalog. That kind of interaction is one reason I keep going back to a site. These guys were determined to get me exactly the product that I wanted.