Ebay doesn't sell anything but a service. It's much like a flea market that rents space to a tenant. It's the tenant that you need to check out.
I generally won't buy any gear off ebay unless the seller provides a guarantee. I bought a lens for my RB67 that was claimed to be 'free of fungus', and the seller posted a money-back guarantee. I bought the lens, and the inside surface of the front group was pitted with fungus etching. It was like looking at an out-of-focus photo of stars. I contacted the seller stating this issue was a fatal flaw (meaning the lens is essentially useless for serious work), I received an RMA with a pre-paid shipping label. I boxed it back up, dropped it off at the PO and 4 days later received a full refund.
There's times I will 'take a chance', especially with something that's hard to find even on ebay. My Tair 135/2.811A, for instance. They're scarce as hens' teeth in the US, so when I saw one pop up with a lower-48 seller, I jumped on it even with no guarantee. Sometimes you just pays your money and takes your chances.