Well as I've previously stated my gear is around kara/T4...so I guess that those items won't last as long as T5/T6. Buying and selling gems...hmmm...keep forgetting that I can do that. I might give it a try. After all, I leveled fine in BC with crap gear so I'll manage to do so with WotLK as well
At this point there is not much use in buying gear upgrades imo, at least not small upgrades. As stated above at lvl 75 you can do an arena quest like the nagrand one that gives you a 1h mace that is a straight upgrade to the mace Kil'Jaeden drops. If you have some really crappy items in one or the other slot, you could upgrade that, but S1 mace is surely good enough to get to lvl 75, most likely you will pick up a green weapon as good as gavel on the way anyway.
I replaced my last piece of T2 at level 77, I believe. Not _that_ many people have T6 or equivalent when you consider the entire player-base. For most of us who haven't raided for a long time (which is probably not that many people, actually), even on our mains our gear is pretty crap. However, I wouldn't worry about whether to upgrade gear or not.
If you upgrade it with badges, great. The only other thing you will spend badges on is gems, which then become gold. And gold is easily spent. It will be handy, profession-wise to go into WotLK with as much gold as possible, but it's really not that big an issue.
So spend the badges however you like. You won't regret whatever decision you make to the point of deleting your toon (I hope).