I think OK.
The seller has a good reputation.
In the event of a dispute, ebay takes the buyer's side 99% of the time.
Use memtest86+ to test and if there are ANY errors, request the seller to give you a prepaid return label.
That said, one 8gb stick is not likely to be a wise choice.
for 8gb, a 2 stick kit of 2 x 4gb is better performing since it allows you to run in dual channel mode.
If the intent is to add to another stick, there is no compatibility guarantee since all ram must be matched.