This is just a bit different than
Is it okay to ask for money donations in an answer?
but not by much. There are a lot of intrinsic motivations for why people contribute their time and their knowledge to this site, but receiving payment or "tips" isn't one of them. While I appreciate the gesture, it really is not warranted or even particularly helpful to this site.
There's also the issue of user solicitations in general (which are not allowed). Asking people for contact information can get ugly fast. There's this whole realm of "I'll send you free stuff for your contact info" spam that would not be allowed on this site. So we just don't allow these types of solicitations at all.
So while I cannot control how you interact with user off this site, I do not think it's a good idea to encourage or allow users to solicit contact information for the purposes of payment or anything else.
Interesting question. Thank you for bringing up the issue.