So you special ordered the wrong frame size? Why would it be the responsibility of the shop to buy back your now used bike and sell it? Just sell the current bike on your own and order a new frame in the correct size. Or tear down your current frame and build up the new frame with the same components then sell the frame that is too large.

Either way- it doesnt seem like the shop's responsibility to take back your incorrectly sized bike. You said that money isnt important- just buy a correct bike and sell what you have.

I have no idea what a home owner's legal advice subscription is, but I would agree that taking legal action against the business would be a nuclear option.