You have to make your own decision about how much the risk of buying on eBay is worth in price. If you could buy a brand new engine for $100 from a reputable company with a generous return policy and a warrantee, how much would you buy the same engine for on eBay, figuring in the potential hassle? Even if you are sure you won't lose your money, you may end up spending a lot of your time resolving issues.

Your decision will be based on how much you value your time, how much you hate hassle, and your financial position.

For me, the max I'd pay for the new engine on eBay is about $60. If it were used in excellent condition, I'd pay no more than $50. If you look at the prices others are willing to pay on eBay, you'll see that this effectively shuts me out of the market.