Im wondering, should i buy weapon in the marketplace or buy weapon from the NPC with lower price tag? i notice some of the weapon being sold in the marketplace comes with a higher quality stat. 

thanks in advance!

Higher quality means higher value in stats(attack, accuracy, armour, etc), like this two weapons below:

Buying weapons from the NPC will give you random quality, nobody knows what quality you will get before you buy it.

If you just want buy some weapons for your sicarios, the market always have cheaper one, since the quality won't affect the sicarios' gains.

If you want buy for yourself, buying it from the NPC if you feeling lucky or want higher quality the the market listed, otherwise market will be better for high quality and relatively low prices.

edit: I'm just being dumb at adding image with wrong link *faceplam*