If your budget for a
new bike is $500 for
two bikes, you are not going to find much worth riding, especially if you intend to ride them for years to come. Bike prices have risen enough that $500 is still in the entry level category for a new bike. Better than department store bikes but still with the lowest end components for a company like Shimano. The good news is that you can get a bike that fits if the store is competent. It's a lot harder to find a used bike that is the correct size, especially if you are at either end of the height scale.
There certainly are some bargains out there in used bikes but you really need to be able to identify which component groups are decent. There are lists online like these for Shimano road and mountain bike component groups
https://www.choosemybicycle.com/en/w...nent-hierarchy and
Ride on the Edge: BIKE GROUPSET HIERARCHY. Try to find something with mid-level components or better and also lightweight. A heavy bike is a dead giveaway for being cheap. The last thing is make sure the frame size is right for you. You can get an idea which frame size you need by visiting a bike shop and finding out which size road or MTB frame is right for you. Be sure to ask if they have any used bikes in stock. You might just be lucky and they have one available.