Advice for Buying Used

Getting ready to buy my first F150, my budget is about $30,000. Looking at used 2019-2023 model years, I see a lot of 2021s at various ford dealers in my area all with 20,000-50,000 miles. Is there any particular reason why there are so many 2021s being sold with such low mileage? Is this typically when leases end or is there maybe known issues with the trucks?

Also with my budget being $30,000, a lot of these trucks are priced $32,000-$37,000. How much flexibility do dealers have with the price on used trucks? If I start with a $30,000 offer are they willing to negotiate or is the price kind of just the price? Any advice is appreciated!