If you take a PC home and try it, it is deemed secondhand and leaves traces on the hard drive of your activity, even if you delete whatever you added since getting it.
Most computers come with an operating system (installed) and are ready for the new user to enter their personal detail and register the O/S with the supplier.
I dare say you would not be able to return some other items like knickers to a store if you don't like them after trying them on. Try returning a DVD/CD after you've had it home and don't like it.