I think the timer between purchases is to prevent exploiting the system. If you had millions of credits to just spend on crap you could basically begin the game with an 0/2 char, give him the money and have him level intelligence by buying weapon parts by the millions and cap his intelligence or boost it to a significant level.

However, that said - I agree completely with your other points. Perhaps barterers could get a lump of XP per cost saving, or perhaps the barter skill would have a sliding scale according to the TL of the item bought/sold? there are any number of ways to do it but yes barter does need some work to improve its functionality to the user beyond just saving money.