I sell all my puppies on limited registration with AKC.
If I fill out the puppy registration, have the buyer fill out their section, and send it in myself to AKC, can't the buyer still register the dog elsewhere? Can't the pup be registered in Canada or CKC (Continental) or UKC. Wouldn't the owner be able to have registered puppies themselves to sell later?
The reason I ask is that I had someone interested in a puppy but they said they would not be spaying her. Red Flags for me. I did not sell them a puppy.

No. They cannot. Canadian KC will honor AKC Ltd, and vice versa.
UKC will not honor, however. They will register full (breeding) even with an AKC Ltd. Same goes for Cont KC, Int KC, all those types of registries.
The pups would not be able to be registered AKC, but would be registered through the other registries.
Kind of scary when you consider that could happen with any one of your pups right here in the US.