If you didn't notice, I imagine that you weren't responsible for disclosure. However, given they arose after your residency, you are likely responsible for at least part of the cleanup. It would be a nice favor to the buyer at least. I would call in a pest control company to advise you on the situation and the extent of the damage.
There are a lot of scenarios where people do have to include curtains in the sale. There are certain things on paperwork that signifies this kind of thing. If you signed the papers the wrong way, you will have to leave the curtains as a part of the sale. You'd be surprised how often this kind of thing actually happens. I hope you can get this figured out.