"I have a question and maybe you are just the guy to have an answer:
I am seeing a lot these days on ebay of '52 Topps cards that are reprints but virtually impossible to tell from the real thing. This is scary - how can we collectors know if we are getting the real thing unless the seller is knowledgeable and honest?"
(Judging from the condition of my own 1952 Topps cards, reprints are the least of my worries. I'm usually glad when they're all in one piece.)
The "reprints" seen on eBay are usually the high-dollar ones like Mantle. Since even a beat-up Mantle can fetch a considerable sum, an unscrupulous seller may feel it's worth the effort.
My answer is going to be different, since I've stopped doing eBay (I haven't quit, I just don't find myself there all that often). I buy my '52 Topps cards in person and have seen and felt enough of them that reprints can be fairly easy to identify. Dealing with sellers I trust probably tilts the scales in my favor as well. However, if I were buying them online and didn't know the seller, I'd look for some things:
1. If the scans appear fuzzy or out-of-focus, I'll assume it's not good or that something is being hidden. Maybe asking for a larger, clearer scan can help. Many fakes have different fonts that will show up in a scan. Of course, if a seller either doesn't respond to your request or refuses to produce a better scan...he's either hiding something or just doesn't want your business.
2. For some higher-dollar cards (or high numbers), it may be a good idea to buy them graded. While many of my own cards won't grade all that highly, there are '52 Topps cards out there that are graded anywhere from A (authentic) to 5-6 that in some cases are less expensive than buying them raw. If you see a good deal, you can always -- carefully -- break them out of their holders if you don't want them.
3. Again, deal with sellers you know and trust. If you have questions about anybody, ask your friends, especially the ones who buy similar items. Online trading groups are good places to start, as are online forums such as Net54 and CU. If somebody's been treated badly, they'll jump up and say so.