IBTL only applies if it's a non-paying member asking the "what is it worth" question. Green members can sell in the classifieds, so a green member posting a "what's it worth" thread is in no way attempting to get one over by circumventing the membership requirement.
I have no idea what an HK 91 goes for these days. Hopefully someone here can shed some light.
And on a side note, if there's a thread that seems hokey please just report it so that one of the mods can take a look. "IBTL" posts really don't help much.