Or not....
You've provided us with no information that would allow for the conclusion that the agreement is not valid. Whether Transformco is a successor in interest to Sears with standing to enforce that agreement isn't something that we can determine without reading the agreement (as well as the agreement you signed with Transformco) and without knowing exactly what Transformco purchased and how that purchase was structured.
In other words, the best possible answer anyone can give you based on your post is "maybe." For $5k, it might be worthwhile to seek a consultation with a local attorney and to pay for a letter argues you shouldn't be liable and, maybe, which offers some sort of compromise (e.g., payment of $2,500) to resolve the dispute. The alternative would be to tell Transformco to pound sand and see if the folks there think it's worthwhile to sue you (presumably in small claims court) over $5k.

