Understood you explanation the first time..... about the + or - $10........and IMO you are correct.
I rarely sell a gun outright..........maybe as described above to a friend....but have been known to "trade" up in local gun shops........
Learned long ago you generally get nothing for extra magazines or leather gear.......but you/I generally don't get asked about extra magazines or offered more if I have a second or third.
I've found dealers to be all over the board on used guns......some sell the extra magazines and leather with the gun ...... some strip them out and sell separability...unless asked or pushed to throw them in for a sale.
So I started to hang onto them...... at least the more common ones or if I'm trading up it gives me one more magazine.
But to TTSH's question.......I think a used gun will "sells" faster to a knowledgeable buyer with 2; or better yet with 3 magazines...... as long as the price is "fair for the package". You may be $10 or $20 out of pocket but the used gun will sell more quickly!
On your other points.....I've always tried to pick up every magazine that matches a gun or type I have; or want, if the price is right......not so much anymore but LGSs use to have baskets of used mags.......cheap. Often they didn't even know what gun the mags were for...... but would happily take $10-15 for them!!!!!!!!!!!!!!!!!!!!!