Slightly adjacent to the topic...
I have sold guns in the classifieds here for my grandpa who is not on INGO and lives in a different city than me. I post the description and pics he provides me and make it clear I am selling for him. I say where I am located and where the gun is located. If the buyer is closer to him I'll set up the introduction, if the buyer is closer to me, my grandpa and I see each other all the time so he'll give me the gun for me to make the sale.
If the 'friend' is a buddy from work or next door or something, the only reason I could see to post that you are selling for a friend would be if the friend was not on INGO and didn't have the means to sell it on their own. It just shows you are being honest about the actual ownership of the gun.