I’ve tried to buy thousands of domain names from registrants who told me some variation of ‘the domain name is not for sale.’ That may be true at that point in time, but the status can change. It doesn’t hurt to follow-up every once in a while.
I recently spent some time looking through hundreds of old emails I sent trying to buy domain names. I followed up on some older leads to re-engage the owners. Perhaps circumstances had changed since our last discussion. I like to follow-up between 6 months and a year after an email exchange, when I see a domain name isn’t being used.