Well, I may be fairly low level, but it's always struck me as odd that a lvl 1 pays the same amount for ammo that a lvl 200 does. (this was rather irksome a couple of patches ago when I wanted to use a bow but the then ridiculous cost for arrows made it a money losing proposition for my low level agent. Perhaps if ammo came in QLs and you needed to have your ammo >/= your weapon QL. Or, alternately, just make some uber expensive ammo that gives a bonus to attacks so high level chars will have something to spend money on (and make this a store only item).
And, as far as taxes go, if they want to implement something like that, they could make it a progressive tax that took a small percentage if you didn't have much credits, but took a progressively larger amount the more credits you have.
As a low level (45 Agent) char with no guild, I dislike the inflationary, ruinous economy which has priced everything but store bought items out of my reach. It took me many hours of mission sneaking just to earn enough credits to buy a full set of implants--there is no way I could ever buy anything decent from another player.