I assume you are doing this already, but research everything about the seller ... in particular, do they own a home, car, etc ... and in general, what assets do they actually own.
If they own decent amount of assets, pursuing legal civil action is likely to be sucessful; seller may settle if backed into a corner.
If the seller appears to own few, if any, decent assets, then you need to see if you can somehow get their bank account frozen - likely require the FBI (I assume from your previous posts the seller is in the U.S.) to get involved ... not sure who can freeze a bank account - perhaps you get a court order? Obviously an experienced attorney will know the options.
Anyways, key is to determine assets the seller has, and if possible try to get "freeze" or somehow gain some "leverage" so to speak. One can't get blood from a stone ... if they own little to nothing, then you're likely out of luck and chalk it up as a learning experience; tax deduction maybe ... FBI may pursue criminal action if pressed, but don't count on that going anywhere fast.
Ron