Some posts make buying a used bike sound more complicated than brain surgery. There is certainly the possibility of getting burned on Craigslist. I agree that the OP, if he is satisfied with a WalMart bike as a first bike, should just go there and buy a new one. That comes with a couple of caveats. First, there is no guarantee that the minimum wage employee who assembled the bike did it correctly. My step son was injured when the front wheel came off a Target bike. I wasn't smart enough at the time to check it out.
Second, some shops will not work on big box store bikes, but they might be willing to take a look at one to insure it is at least safe to ride. My local shop is kind to riders with this kind of bike, other shops may not be. Third, see the second. If you have problems with the bike, WalMart probably won't help you and the local shop might not do so either. Fourth, I think it unlikely that you will be able to satisfactorily convert a WalMart gravel bike into a road racer.
You say you have a trainer already, so I think you should buy the new WalMart bike you like best. Then save your money for a Canyon when you discover you enjoy riding and are ready to move up.