How should I handle money returned for a product that I did not return?

In my opinion, you've done more than you need to do.

Some would argue that you didn't need to do anything at all. It wasn't your fault that payment was reversed and it is the job of the seller to take care of that.

You've informed the seller that you don't believe you were charged for the laptop. You don't need to do anything else. Let them take care of it if they choose to do so.

You may get a free laptop out of the situation, but you've done nothing wrong so you don't need to feel guilty about it!

Open a savings account at an online bank, as a "holding tank" for the money you'd have used to pay for the laptop. Then if the vendor realizes their mistake and charges you again then you'll have the money.

If after a year they haven't reversed it, I'd take that as a sign that they aren't coming back for it.

I basically agree with gaefan, you've done more than you need to. I would say that contacting the company should be enough. But there is one thing missing in that answer, thus I'm writing this as a separate answer.

Do you have any proof that you contacted the company? Something that the company doesn't control, like an email? If you don't have that you might want to send them an email about the whole thing, to get proof that you've contacted them about the issue. Then there's no risk of you getting into trouble about it in the future.

You can also choose to not contact them again, in that case you probably won't be in trouble but might have to pay for the laptop in the future, which could be years from now.