I put in a fair amount of shopping restrictions in my game.
Most of the time that I put in shopping restrictions - potion limits, rolls to find items, limitations on what spells can go on scrolls (or limitations like "no one sells Universal scrolls"), etc. it's not intended as a punishment to players or to nerf the rules.
Mostly, I put in those limits to speed up gameplay by reducing two things:
- shopping time
- if X then Y planning.
By shopping time, I mean time the players spend looking for items to eke out just a bit more power for their guys. It's a sensible action to take, in a meta-game sense. Spend that money to be a better delver. It's a little less so in-game, since what's the point of delving for money if you spend close to 100% of it getting better at delving? It's a casino where you can't cash out your chips so you may as well keep betting them. But in any case . . . it takes time. Flipping through books. Asking the GM. Rolling for stuff. Asking other players if this purchase is a good idea. Hitting the books again. Flipping through every single spell you can look at to see if might be useful. This inevitably takes time. I'd like to spend this time gaming, not prepping for a delve. It's not possible for the players to do this all ahead of time.
By If X then Y Planning, I mean "If you can get a scroll of X, we can try Y." "If we can afford X, we can use it to do Y." Then we cut to more of the same as above - pricing things out, more discussion, more pricing, more discussion, sometimes more rolling, etc.
It's fair to say that my own rolling restrictions - not all items are available - feeds into the uncertainty here. Irony, I know, since it's intended to cut this down a bit. It does cut down on the first (roll, it's not there, move on) but not the second (think, plan, roll, then decide.)
But in general my approach is to attempt to get more actual play in, at the cost of shopping and prep. Adding restrictions has aided this but not eliminated it. Leaving them off, originally, made for a lot of shopping and prep time. It's a question, to me, of the right amount of restriction to keep in some utility for money without making rewards from last session eat into the time of this session.