Selling existing plots safely - is it possible?

Since plots are permanently tied to a wallet, it seems like there may not be any 100% secure way to sell existing plots since it’s impossible to prove that the seller has destroyed all copies they held of the private key. Has anyone found any interesting solutions to this?

Some ideas I’ve thought of which may or may not be viable:

  • Someone mentioned simply using a legal contract to enforce the sale rather than relying on technology - I’m not sure whether this can really solve all the problems
  • Chia also supports smart contracts. Is it possible to create a contract that will automatically transfer any farmed XCH to a different wallet? Is this sufficient to prevent a rogue seller from simply publishing a new smart contract that transfers the XCH out first?
  • Can a wallet be converted to multi-sig after it’s been created?
  • Is there any other way to securely pre-arrange to transfer XCH out of the farmer’s wallet?