Does the used vendor have a hopper capable of giving change back for multiple $10 or $20 bills in a day, or allow for multiple purchases for a bill deposited before dumping the rest as change? Assuming the answer to both questions is no as is the case with almost all vendors, you won't even be able to take bills larger than $5. It seems like a mostly moot point in your case anyway since you have employees on site most of the time and can give change.
We have only drop-shelf vendors, quarters only (They accept dollar
coins but I've found only a couple in them in the last six years), and customers will frequently change a $5 and spend it all in
vending. I very rarely see anyone spend more than that at one time.
Credit card acceptance seems a lot more important to gaining more sales than accepting larger bills.