Yeah, the idea is not bad, but it has some potential ways to abuse it. How? It's simple - one could sell his/her almost fully deteriorated weapon (for some caps), and then... voila! It's magically repaired by merchant, and you can buy it back, fully repaired... It would somewhat more ruin in-game economics in my opinion.
I think stacking (and unloading of ammo) weapons of the same class and deterioration level would solve problem just fine. No need for repairing it.
Besides, if merchants could repair weapons/armors, what sense would be to use of Repair skill (other than dismantling weapons/armors, some gathering needs and gaining access to some professions)?
If you want more time to barter, invest some points in appropriate skill - it's simple solution, and available to anyone. You don't want to give points in Barter? Then you have to live with that. And with short barter timer - it's your choice.
Simple as that...