think of it this way....
Would you take out your seatbelts and replace them with long pieces of fabric your Mother stitched together for you? No.
Why take out a stock seat that has been specifically designed to keep you safe in accidents and replace them with products of unknown quality/origin? I can assure you none of the vendors on ebay have the money, facilities, nor desire to crash test any seat they sell.
Sparco, Momo, Mazda all do. They make customer safety a huge aspect of their business. eBay vendors make colored and bold headlines a huge aspect of theirs. You make the call.