They do because they are cheaper. You will find that there are plenty of rims and tyres around because people upgrade.
Are they safe. Don't see a problem, as long as the rims are free of rust, not buckled etc, and the tyres have good tread and are not re-treads or re-grooved. Remember that your CT wont do the same k's as your car.
On tyres they have a life of 4 years, so check how old they are.
A tip though. Try to get the same rims and tyres as on your car.