There are three ways you can get hosed on
ebay.
1) Seller non-performance- the item just doesn't get shipped.
Check feedback, use your gut. If they've been doing this for 19 years and have 99.6% feedback, you're fine.
2) Inadvertent seller fraud- the seller unknowingly sells a fake or counterfeit coin.
Caveat emptor. Stick to
TPG for ANY key dates or reliable sellers who would likely have a Sigma for bullion.
3) Deliberate seller fraud- the seller knowingly is selling fakes.
Don't buy from the account that has 43 feedbacks accrued in the past 2 weeks, all for .01 cent items, trying to sell an unslabbed 1909S-VDB in mint state condition for $500 that he '
inherited from his uncle'. Don't buy the $15.99 American Silver Eagles. No, the guy in Russia really doesn't have a trove of circulated 1948 Canadian Dollars, they're forgeries. Common sense can go a long way, but every now and then you run the risk that a credible seller is going rogue, or 'dipping their toes in the water' of fraud, ie, most of the coins they sell are legit but they salt in a few fakes knowing they'll get away with most, then just apologize and plead ignorance when they get caught. Very little you can do about the latter.
I will say this- just don't buy key dates that are not slabbed, period, and compare the number with the picture.
You can buy high quality struck fakes of most every key date from China pretty easily, more sophisticated fraudsters use the darknet where there's a full array of high quality fakes, fake slabs and labels, holograms, etc. Its a minefield out there. In my opinion, the fake issue is worse than people are giving it credit for and a lot of people who believe they have such great expertise in spotting fakes they're immune, have fools confidence. Some of these fakes require a credible, specialists eye.