I agree with all of this, but the added factor about "Mike" knowing some info about the bass that the seller didn't know is what makes the deal kinda shady. If the story as presented is true, then "Mike" had some insider knowledge that the bass was worth more than the seller thought, and then withheld that knowledge for the purpose of making a profit. It certainly isn't illegal, but it is immoral. I don't know how else the seller could research this, since they were giving it to the original manufacturer to sell he would think that they would tell him about anything unusual about the bass that would add value. Especially seeing that he was under the assumption that the bass would be sold with an 80/20 agreement, he would think that Fodera would do all in their power to sell it for the highest posible amount to give them the highest posible 20% profit.
On the other hand though, reading what happened, it sounds like "Mike" knew exactly what he was trying to do when he said "I have a buyer for the bass, the deal will give you $6200". He was careful with his words there. The seller should have asked "so that means you're selling it for xxxx, right?" Or he should have asked how much the bass appraised for. If I was the seller I certainly would have asked more questions or got certain things in writing when conducting a deal for this much $$. If he would have asked a couple questions, "Mike" would have either been caught in a lie (and the seller might actually have a case in court) or he would have had to give up the plan and tell the seller all the info.
This situation is like someone taking a beatup Fender jazz to a music store, without knowing the value (maybe it was a gift from a relative that passed away) and the store giving them $100 for it, while knowing that it is a vintage 1962 jazz that they can sell for $3000. Not illegal, but certainly immoral.