You can sell drops to NPCs to slowly build up zeni to buy extra stones off AH.
Do you know how much u70 purples go for or even pinks? That would take an absurdly long time to even get +6 on a lvl 70 weapon if someone used your strategy at a low level trying to gear for end game, and thats not even considering if they want a higher + like 8-12. Why not get to end game and actually grind drops where you can potentially make way more money than just selling rubbish drops to NPC's and then you could possibly get u70's to drop without having to buy them? It seems you want the game to be even more grindy than it already is.