Hello!
I sell cars for a living, New cars at one of the Northwest's largest main dealers. I do around 85% of my business on finance deals and can't remember the last time I provided an agreement with an APR of above 9% or sold a car anywhere near to list price. I also have a good percentage of my business with people who live in rented properties and find that this alone does not affect the ability to obtain finance, the interest rate nor the selling price of the vehicle. There are a lot of main dealers around the land who think they know the job and have a strange prejudice, deciding for themselves whether a customer can or cannot afford the car, hence the jaded response from the dealers you have visited.
Used car salespeople need no excuse to 'rip people off' especially at ultra competitive (for salespeople) CarSupermarkets. They are often paid on Metal commission, Finance commission and finance penetration so the more finance deals they do, at as high a rate as they can get away with, the more they get paid. Be aware of "Drive away in an hour, bring your I.D", this possibly is to guard the dealer against 'buyer's remorse'. They want you out of the showroom within the hour, possibly so you only notice the APR when you get home and it's too late.
Research the price of your used car, making sure it is fair taking into account mileage, age and condition. Don't deal if you cannot find the APR out before agreeing (I'm sure the FSA would be interested in this practice...!). Check the price of a new equivalent, not just this but a finance deal, the used car may have a lower screen price but interest rates are higher and the overall cost is often more. Make sure you aren't being duped into signing for 'compulsory' PPP, raising the cost, interest rates and bank balance of the salesman.
Hope this helps a wee bit...
Jon