Since your title question asks about eBay, here's my story purchasing on eBay from Chinese sellers, take it for what it's worth.
I bought a 36v LiFePO4 kit off a seller from eBay. Battery was advertised as 10Ah. I already had a motor, I was just looking for a battery and a controller. Parts arrived, installed on bike, went for a test ride. Battery died after 8 miles @ 15mph with significant input from pedaling rider (me).
Took battery apart, individual cells measured wildly different voltages after a full charge; anywhere from 3.97 to 4.23v. The cell connectors were poorly soldered and, in two instances, cracked. I fixed the solders and replaced the bad connectors. Still no go; the battery simply would not balance.
I posted a few pics to the tubez. Battery was, in fact, not LiFePO4, but Lithium Manganese and apparently had been sitting in a warehouse for over 5 years. Capacity was estimated around 5Ah. By the time all this was figured out, eBay seller was long gone (no longer a registered user) and I was out $250 for useless junk.
Next eBay experience was buying an RD32II camera for my bike since I seem to regularly have close encounters with cars. Camera arrived but would not take a charge. Seller said "Sure, I'll take it back, just send it to this address, but please only declare the value as $10 so I don't have to pay taxes." It cost me $30 to ship a $150 camera back to China. 3 weeks later, no word. Filed a PayPal complaint. Now this is completely my fault, but I relied on the customs form for tracking and did not opt to take the $14 extra tracking. Based on this, PayPal denied my claim. Seller claimed the package never arrived and never answered a single email from me after providing me an address to send the item back. *Poof* There goes another $180.
So, you may have an awesome experience buying off eBay from Chinese sellers. I didn't and I'll never do it again. I buy local if I can; or through a reputable Internet dealer. I may pay a bit more at Grin Cyclery
, for example, but in the long-run, it's worth every penny.