IDK if it is genuine or not. But what happens if you try to return it (a fake) you paid cash, and the guy says you switched coins.
I'm surprised that so many people are thrown by the cash only policy. My favorite coin store has been cash only for about 7 years now and honestly I kind of expect it from coin dealers. I've also been to shops where they'll only take credit if you pay extra to cover the fees. I believe they do this because most of their business is bullion and it ruins their margins. I also think more coin and bullion people than the general population tend to be private when it comes to their financial transactions. I wouldn't let this policy bother you.
Indeed. The taxman is another reason, but still, it's perfectly common and in no way the automatic red flag some have made it out to be.
Personally, I think it looks Authentic... comparing it to the mint marks shown on PCGS - https://www.pcgs.com/News/Counterfeit-Vs-Authentic-1909-s-Vdb-Lincoln-Cents It looks like Mint Mark #2. I agree that it is probably Improperly Cleaned, but the overall grade to me seems to be about F-12 (hard to tell with the Angle the Obverse is photographed at.) NGC Retail Value is $875, $815 if you drop a grade... not sure the GraySheet value, as I don't have it in front of me. $600 doesn't seem like too bad of a deal to me.