I buy about everything on eBay, even pearls, sometimes.
I recommend you look at a site like Pearl Paradise to get the most honest prices for a type item, then go shopping elsewhere, including eBay. Few eBay sellers are able to sell the same quality for the same price, because PP has developed connections with good pearl factories so they can offer the best prices anywhere for high quality pearls. So check your honest prices, then go shopping on eBay. Anything less than PP's prices and you know you don't have the same quality, because PP's quality is in their mass buys that no eBay trader can do.
The factories in China sell the best quality to the biggest buyers first, so what is left for eBay sellers is far worse quality for the same price.
Having said that, there are a few trustworthy eBay pearl sellers, but their prices are not as cheap because they sell quality. Drusy Designs and Augustus Pearls are two of them. Sven of Augustus is active here and we have come to appreciate him as a great source for genuine south sea and other pearls. Carolyn of Drusy is a member, but not active, though several of us have known her for almost a decade.