If I'm buying, I bring my own halter and lead because I know how stingy some people can be.
If I'm selling, my horses all have their own color coordination thing going on, so I'll usually include both halters (because I have a rope and nylon halter for each one) as well as a lead. Also, if it's a new owner, I'll go ahead and include a couple of brushes and some stuff that a new owner probably wouldn't think of (cheap things, of course) because I want those horses to be well taken care of.
I had no idea it was a law in some states, though. Good to know. :)
Red Money Maker (Red) - 2004 Sorrel QH Gelding