I’d be in the same boat as Dominic. I had a home I was trying to buy in a park and the seller was adamant she deserved the money she was asking and the park owner was adamant about receiving the lost rents from her not paying. Neither would adjust so I believe it ended in an eviction on her record and him having lost 5 months lot rents, eviction costs, and now a dumpy trailer he has to flip, sell for a few hundred, or trash all at his cost.
I think you just need to eat that cost - did a security deposit not cover it?
Your rules should include that all offers on a home be conditional on park owner approval of buyer. You need to screen any interested buyer and approve them before they can be accepted as a resident. All offers to buy must be conditional.
If a tenant is in arrears of rent you do not approve any buyer until all debts are caught up. If the sale is already completed without the buyer being approved as a resident they are not permitted to live in the home. It that situation they may choose to pay the arrears if they pass screening.
I usually take tenants to small claims court, it is a simple process and not expensive.