I don't see a problem doing so. It doesn't bother me when I get such requests (no harm listening) . . . and I've sometimes ended-up selling!
Nothing ventured . . .
That's good to hear - sometimes I feel bad about cold calling someone who says NFS...
I think it all depends on how you make the request.
When I do it, I usually write something along the lines of: "I know your art says NFS, but if you ever change your mind, please let me know." and I drop a comment on the piece and add it to my favorites, so the owner can easily find me again in the future.
It didn't work for me so far, but I keep hope.
I also got a similar request. I didn't sell the art, but gave advice to the person on how to find a similar one (it was a not very famous indie artist).
I actually got quite flattered that someone was interested on my art.
Of course, for collectors of exceptional pieces who receive the kind of requests daily, I can understand this could be a bit annoying.