Before I picked up my first horse, I went out and bought a (cheap) halter, assuming that he wouldn't come with one. I was pleasantly surprised when the seller told me that he always sold his horses with halters. Scout came with a hodgepodge of tack (none of it fits him, then or now...), but I wasn't there when my family picked him up. Sis bought him a new halter as a "stallwarming gift", and I never saw an old one... now I'm curious, I'll have to ask her if he came with an old halter or not!
I've never sold a horse, but if I did I'd probably go the extra mile and buy a new halter to see him off with. Nothing fancy, just a simple, serviceable nylon web halter. $5-10 at TSC.
I've never heard of laws requiring sellers to provide halters... just seems like the common-sense, neighborly thing to do to me. I'll have to look that up in my state!