The god-awfull shop defiling usleless bead of pointlessness is AWESOME!!! We have documented proof of it being a major cog in wrecking face in unbeatable dungeons!
However, I really have no clue about what I'd put in a veto slot if I had one. Really. I'd probably try to rig one of the item "sets" to always give me the best stuff, but what exactly is the best stuff would be anyone's guess. My pet name for the Blue Bead is living proof - that's what I used to think about it, and now I think the complete oposite.
If I was going for just pruning out stuff with cost-efficency issues I'd probably go... Hmmm. I'd probably... Hmmmm... I'd...
Ok, let's take a different approach to what Darvin and Avatar said, except it also agrees with them. I'd keep stuff I want to extensively test IN rather than locker it OUT. So the stuff I'd be vetoing would change a lot, with most of it being items which are sort of confirmed as "fixed" or rather "won't be fiddled with by devs any further".
Scenario 1 - ELITE: Veto all elite items except the Alch Scroll and the Guitar. Alch scroll is all kinds of stupid, the guitar is a nice non-commital thingy and this would make Elite items a prep I'd be able to use with my regular Compression scroll routine. Possibly leave a keg or 2 in there, not sure. This would work if the Alch Scroll is indeed an Elite item (not sure), if not, then veto everything except the Guitar and a specific thing I want to see.
Scenario 2 - BASIC: Veto out the +10 HP necklace (I only really need a bunch of potions and slayer wand for early levels), the Spoon (not entirely sure on this one), possibly the +2 mana necklace (+2 mana not worth an Inventory slot, although, again, not sure)... Anyway i'd try to get as close to only having the Tower Shiled, Bloody Sigil, Troll Heart and Fine Sword in the pool (maybe I've forgotten something). And I'm not too sure about the fine sword either. The other three supply stuff you can't easily find elsewhere.
Scenario 3 - QUEST: This is problematic.
It's the largest category, with the biggest variety of stuff in it, and also with the most stuff that needs testing / figuring out / tweaking. If I locker stuff I'm not using for convinience - I shoot my betatest in the foot. Right now that would mean Vetoing Fire Heart and Crystal ball (no CP - lujo no buy, also CB does have a really silly effect but 5 gold per pop really puts me off), The Piety Charm (which is hella strong, but 18 gold, again, I'm a cheapskate).
I'm not sure how to use my veto slots (if I had any) to either prune or tailor my Quest item prep and set up an embargo. I'd have to see what gets lockered and prepped, and then see what needs to go out of the pool to make it cheezy.