Sometimes by the time a seller responds to your letter the buyer has already moved on by then.
You can tell the seller that the buyer has already moved on, but you would like to preview their home anyways because you have a few other buyers that may be a good fit for their home.
Here is how the conversation could go:
Seller: Hey Bob, I got your letter about a buyer being interested in my home, I would like to meet them.
You: I'm so sorry Jane, but that buyer has already moved on, but I do work with a lot of other buyers and I think 1-2 of them may be a good fit for your home.
Could I come by this week to check it out to see if it would be a good fit for my buyers? How about Wednesday at 6:00 PM?
This should be an easy call, you've already proved that you work with qualified buyers. Most sellers will be happy to meet with you.