I think (and I should have said so) I was mostly responding to the categorical-sounding 'No' at the start of your comment - such transactions do occur, occasionally even for straight up 'services rendered' (with the other party generally having to pass the 'qualified investor' bar).

I'm probably in violent agreement with you that there is no sensible way someone's going to be able to offer some magical web-based marketplace for such exchanges, for all the reasons you listed.