what sites to use when checking history of used care FREE?

Nothing is FREE......

There is no site that it "good".  No one knows anything except the owner.
Your best option, get the car checked by an independent mechanic. 
It's best to pay the $100-$300 for this instead of going the "free" route.

Good mileage depends on the year and make of a car.  Not the price. 

Example: 
2018 civic being sold for $29999
Has only 100 miles on it.
** great mileage
***** scam price!!!!!

they are only free if the car dealership offers them on the cars they have for sale.

Any site with information of value is going to charge for that information because it costs money to gather and compile it.