When scanning through sellers you don't know, it's easy to figure out what kind of dealer he is. Find a coin that's easy to value and use it as an indicator for whether you should buy anything from that dealer. I know this is a "no duh" kind of thing but I ran across this dealer and thought it could be fun to see some other examples.
The "buy it now" link you posted doesn't work! I must have that coin! Maybe it's a double die reverse frosted triple proof VAM with a planchette error?
This one doesn't even need a coin guy for major red flags. Anyone who "discounts" 73% is a charlatan to begin with. If you can "discount" an offering more than 25% or so, you were asking too much to start. Or advertising a greatly-inflated price you never actually put on the item offered. Either way you're a charlatan.
I see over-grading but with supposedly steep discounts, often, which is similar to what you're saying. I'll try out your method but I'll do it with a seller who seems to be selling a coin for an "undervalued" price for a lustrous "MS" coin. Unless the coin is supposed to look like that... Right on the featured listings, this 25C 1918 Netherlands coin. NGC says MS60-MS63 is $35-$75. Yet, the coin is being sold supposedly for far below that. How about this guy? Would you deal with him?
I suppose coin dealers like that must make enough money to keep them going. And how many buyers who find out later how badly they got screwed throw the hobby out the window and tell everyone they know how crooked it is?
Seriously? I'd only have a coin like that if I got it for .50 cents at the bank and would never even consider paying melt for it much less $67.50