Well, selling on FB is an absolute no no. I doubt he'd attract dog fighters with that price, they normally look for free to good home dogs but no price would put off a puppy farmer. Please say he's not KC registered and being sold with papers? He's future is much bleaker if so, although lack of papers will not put off all puppy farmers and the suitability of people willing to buy a dog off of FB is questionable at best.
All responisble owners, the kinds you would want to look after your dog, go to reputable breeders or rescues. I adore dogs but I would not take one off FB even if you were paying me to. The poor dog could end up with anyone. People lie. They're not going to rock up on Dad's doorstep and say "Hey, can I buy your dog? I'm gonna keep him in a shed in the yard, breed from him continuously, even to aggressive bitches who might cause him serious injury, which I will not treat. I'm never gonna walk him, I'll feed him sporadically and once he can no longer stud I'll kill him with my bare hands" They're going to make out like they're from a family home and they want a pet.
WRT being able to keep both dogs, yes, he could. It is imperative that both dogs get nuetered. A lot of vets have finance options if paying up front is not an option. Some rescues might help him and I'm not sure if the PDSA offer reduced rates. He'd have to ring round places and ask.
I can guarentee the younger dog is not trying to dominate. He is either trying to play or impregnate the bitch (hence speying being vital. Inter bred puppies are not healthy)
Training can help here. Every time the younger dog tries to initiate play he shoud be quietly removed from the room until he is calm and quiet. The bitch should have somewhere quiet to go to where the dog cannot get at her when she wants some peace. One option would be to put a magnetic dog flap on an internal door (to a utility room maybe?) and only give the bitch a magnetic collar to operate it, so that she can escape easily.
There is no reason why the younger dog could not live with other dogs. I have a 'dominant' dog -read playful and enjoys dry humping- and he lives happily with other dogs with a little training and the right other dogs. He now knows if his dry humping is incessent he will be taken out of the room until he is ready to behave like a civilised animal 