I keep buying used dryers and none are drying, I replaced all venting etc.

Gas or electric?

A clothes dryer is just like a hair dryer - Heat and airflow. Do you have heat? I will presume you have excellent air flow. If your dryer is electric you need to make sure you are getting 240 volts to the dryer. If you are only supplying the dryer with 120 volts the motor will turn, but you are not going to get enough voltage to the element to make it work properly. Check for power going to the dryer.

You said they were used. Are you buying these from private individuals hoping they will work? Next time you go to buy a used dryer ask to see it running before you buy it.