Some ideas:
1. Figure out a way that I can just drop off my stuff and you'll take care of renting it out for me, over and over again. I would love to make some extra cash for my stuff but I really don't want to spend the time and pain of scheduling, driving to meet the rentees, waiting for them to show up if they're running late, dealing with it when some of them invariably flake out at the last minute, ect... I would approach local consignment shops and see if they'd be interested in being intermediaries where all they have to do is act as dropoff/pickup locations for your customers. In exchange, you pay them a either percentage or a flat monthly rate.
2. An alternative might be a sort of rent it forward strategy, where instead of returning an item to the owner the rentee holds onto it until someone else rents it, and then passes it along to the next renter.
Of course, both of these ideas make it much harder to gauge whether the items are in approximately the same condition as they were when initially rented. And, at least for idea #1, you'd probably want to add a clause that any item not rented out within a month needs to be picked up, else the renter will be charged a storage fee.
3. Look on craigslist and ebay for professional sellers and ask if they'd be interested in renting out some of their stuff on your platform instead of selling it. That might be a good way to populate your inventory in various cities quickly while making them more money than they'd make by simply selling the stuff. You might also ask local thrift stores, flea market sellers, and consignment shops if there are any items they'd like to rent out on your marketplace.