I’ve been on the other end of the rental business, and here’s some things I would keep in mind if I were the renter:
even “primo” gear gets used hard and even abused (shocker, right?) by people who don’t own it, tents come to mind. Holes from campfire ashes in the rain fly, dog hair and mud inside, food particles and stains, bent/missing poles, rips and tears, and mildew from being stored damp are things we very commonly saw on returns. The only way to determine if a returned tent is in top shape is to set the thing up completely – rain fly, footprint, and canopy – to verify it is ready to turn around again. During busy seasons, it might be hard for renters to do this prior to turning a tent around again. Unfortunately, not everyone is forthright about the condition of their returned tent;
personally, I find it creepy to rent sleeping bags. Washing a bag after every use isn’t really practical, as it significantly decreases the usable lifespan of the bag. We used to provide liners free of charge, but many people chose not to use them, and it was not uncommon for bags to be returned smelling pretty funky, to put it politely;
rental stoves weren’t too problematic, but we did not fire up each stove after a rental return. Once in awhile there were problems;
backpacks seem to be the most reasonable thing to rent, as it’s pretty easy to judge overall condition. The trick is to get a good fit, esp for kids and women, but unless shop employees know how to fit packs it might be hard to get a comfortable fit.
Mark’s comments are spot-on. Good advice there, particularly the bit about checking out the gear before you leave.  Set the tent up, make sure zippers on the sleeping bag work, make sure the stove operates, and load the pack up to make sure it carries comfortably.
We made money on rentals, but it sucked a huge amount of space and labor to make it work. Ultimately, our shop opted not to rent camping and climbing gear and limited our rental offerings to more space-efficient and easier-to-maintain bear canisters, trekking poles, and snowshoes. It was always interesting to check in gear. Some of the things we recovered were money, condoms (both in and out of the original packaging), clothing, and uh, pharmaceuticals.