I think schools handle them differently...some use them as fundraisers, others just put them on as a "learning experience" for kids...depends what you are looking to get out of it...
We do not use them as fundraisers; they are considered an "enrichment event", a service to our kids...
We buy our items from a nearby dollar store, that way returns are very simple, and everything is $1, so running the shop is very simple. We set ours up in an empty classroom. Teachers sign up for a time slot, which we have scheduled over a four day period. We send home forms, asking that family members get checked off that need to be bought for, plus one dollar for wrapping, if desired ($1 pays for all gifts to be wrapped), then we gather a group of volunteers to work shifts at the "store".
It's a great experience for kids, who feel empowered by the ability to make decisions for themselves, as well as rewarded, knowing that they've "bought" something for a loved one....