good points all around, and as some have said the main thing holding you back now is the time.
One option might be, in talks with Mark at A & A, to have the weapon examined by a third party. Someone completely neutral. That way Mark will know he is getting a gun he can sell with no headaches, (immediate and in the future) and you can receive in-store credit to an agreed upon value toward the purchase of another gun/parts/accessories/gear.
Everyone should be happy, I would hope.
Either way I wish you all the best. Its a close knit community, so for all concerned an amicable resolution should be found.