all this ^^^
and, on top of that, don't be a flim flammer, tire kicker, waffling, time pirate.
don't offer half, that's just rude. and be ready to pony up the dough if the deal rolls. none of that "i gotta go to the bank" or "let me move some money around" or "i get paid tuesday" horseshittery. money talks and bullshit walks, amigo.
also, be realistic. are you really going to drive 4+ hours to go get a motor? if somebody doesn't explicitly state in their add that they're willing to ship, don't nag them about shipping. and if they say they're willing to ship, the rest is on you to figure out. after it's on a pallet, it's not their problem.
on top of all of this, you don't even know if your hypothetical new car is even going to need a motor.