The main thing to watch out for when purchasing a used unit is the battery, since that can be a big ticket item to replace, depending on the
scooter, the model, the manufacturer and the battery type/chemistry.
Rental scooters are (to use an expression related to horse training) "rode hard and put up wet". Which means that like rental cars, they tend to be abused. Batteries are often not charged properly, in accordance with manufacturers instructions. People can (and will and do) drive them off of curbs, let them sit in the rain, bash them into doorframes and inflict any number of other indignities upon them. You may see large scratches or dents on the bodywork. Tires may have big chunks out of the tread. Seats and/or armrests could be ripped or damaged.
So, *if* you are determined to buy a used unit, you need to do a bit of due diligence.
First of all, NEVER buy a used
ECV sight unseen. I don't care what they tell you in email/chat/over the phone, NEVER buy a used mobility device without seeing it first in person, and without checking it over thoroughly.
Get in writing that if the battery does not perform at new levels (here you will need to research, and see what the manufacturer says the run time should be on a fully charged battery for that exact model) then the seller will replace the battery with a new one at their cost, and in a timely fashion.
You must drive the device, and make sure that it functions properly. Check all controls (Does "turtle" setting really slow it down? Does "rabbit" setting really speed it up? Do switches work as they should, or do you have to "fiddle" with them?) and check all functions of the device (does it run straight, or does it tend to veer off to one side? On smooth level pavement, is the ride smooth, or does it still feel bumpy?) Can you turn a tight circle to both the left and the right? Does reverse work?
If there are headlights/taillights, do they work? If a key is required, are there two of them? Is the owners manual plus the original charger for the unit included?
If cosmetic problems don't bother you, then you don't need to worry too much about body damage, (although excessive damage will indicate that the scooter has been abused) but seats and armrests do take a lot of wear and tear, so make sure that they seat is both comfortable for you, and in good condition.
Some rental vendors switch out their equipment, and sell off the "old" rentals every six to 12 months. Others sell them when they are beat to pieces and no longer viable to keep running as a rental. Anyone willing to sell a fairly recent rental for the price of 2 weeks rent may be looking to dump off problem units. You may be better served by checking prices for an identical unit on
Amazon, to see what it sells for new. You may be surprised.