A fool and his money.... Kudos for not being the fool in this case. There's little more dangerous and/or damaging to this hobby than a newbie on the prowl for a "deal". Unfortunately, many do not do as you have and seek the assistance of others. That said, I have to respectfully disagree with Claw but only to an extent. When or if one truly knows what they're doing, there really isn't a venue that should be avoided unless time is the issue. In particular, Craigslist once was a great coins venue but oddly enough went way downhill only after silver peaked some years ago, almost as if the clowns were desperate after missing the gravy train. Once this happened newbies trying their luck there were akin to meat in a freshly chummed shark tank, and in many cases were specifically targeted. As for Etsy, there does seem to be an awful lot of junk there, but doesn't mean there's no chance a gem will appear. Knowledge along with a willingness, even eagerness, to do the work can pay off. Unfortunately, it usually happens when least expected.
And conversely, error coin collectors typically want a nice example of the error (off center) are aren't willing to pay a premium for the key date/variety so they usually have no interest in them. What you have to find is someone putting together a complete date/mint/major variety set that wants every coin to have a mint error. Such collectors are rarer than the coins.