Although you are correct regarding the legal definition of fixtures, since you signed a contract in which the window coverings were designated fixtures, you would be bound by the terms of the contract.  Although it is NOT possible to predict how a court will rule in a particular case, most likely the court would uphold the contract in which the window coverings were designated fixtures.The buyer would have to prove damages in a potential lawsuit against you for breach of contract.  The buyer would have to prove that the fixtures are worth $200 instead of the $100 that you are claiming.  The buyer would also have to mitigate (minimize) damages.  For example, if she could obtain a replacement for $100, she could not select the most expensive place she could find to replace the window coverings.  If she did that, her damages would be reduced accordingly.If the buyer were to sue you and win, she could also  recover attorneys' fees; however, if this case just involves the cost of the window coverings, it would be a Small Claims Court case where there would not be any attorneys or attorneys' fees.  The buyer could recover some nominal costs such as the court filing fee and process server fee in addition to the cost of replacing the window coverings.