It would be a great addition to item comments allowing @username notifications so buyers can help and talk amongst themselves with notifications ala github, slack, etc…
+1 here, that would be great!
Would that be a good idea?
Aside from item comments not being intended for this and potentially becoming like a micro support forum, my main concern would be that there would be no protection for people or control.
There are some people who feel that buying a theme and changing some text and images constitutes being a developer and may, even with the best intentions, provide inaccurate advice which could create knock problems.
There would be no recourse and long term could well end up (rightfully or wrongfully) reflect badly on envato or authors. .
I am with @charlie4282, do you think it is good for all users as they can tag anyone they want? and do you really want to create yet another small version of support thread?