Although used tires are significant cheaper than new tires, Consumer Reports states that the safety risks make it worth spending more for new tires. Used rims, on the other hand, are often replaced when drivers upgrade to new rims, and their simple design makes them easier to inspect.
Most drivers stick with their tires until they need to be replaced, so most replaced tired are disposed of or recycled. In addition, tires inevitably wear down as they are used, so used tires don't last as long as new ones even if they are maintained well. While it can be difficult to detect subtle signs of wear on tires, rims are fairly straightforward, so it is far easier to determine if they are safe to use.