I work at a dealer and I'm the guy in charge of the used vehicle prep so I could go on for hours but i'll try to keep it short.
If you keep them, the next owner of the van will be the guy that gets shafted and not the dealer, unless the next owner makes a big stink about it. I also wouldn't be surprised if they ask you for them, or even call you a few days later to see if you accidentaly forgot to turn them in. Obviously by that time they won't be able to MAKE you give them up but if it was an accidental omission a lot of people will bring them back in for us. Most likely the dealer will bring it up right after they test drive it. But it's a no brainer that the dealer wants to put as little money in it as possible, so if you do keep them, they won't buy another set unless the next owner raises hell, and most won't. Half of them get sold to older people that like the height of the van so they can get in and out easier so they won't use the player anyway, and have no desire for the headphones. Another note is that if it's a dodge/chrysler there ARE two different styles of headphones and they will not interchange. Found that out the hard way after locating a set for a customer and they didn't work and we had to locate yet another set.
Basically if you are looking at a used vehicle and something is missing, it's about a 95% chance that it's gone because the previous owner kept it/fogot to put it back. Contrary to popular belief, the dealer doesn't stockpile loads of useless crap that they removed "just in case". It's too hard to inventory so if you did need it you wouldn't know you had it or where it was. What the hell would we do with a set of floormats out of XXXXX whatever? Keep them to give to the next random customer IF we ever happened to find another vehicle the same model/color, when the vehicle is 5 years older and no one cares about floor mats? I suppose there are some dealers out there that would keep them and try to SELL them to you after you bought the car but that'd be too obvious and people would see right through it. The bad rep. wouldn't be worth the $25 or whatever. Same thing with key fobs. New keys with all the integrated chips and such are expensive, so if we only get one in we won't pay for another unless we a have a customer that is raising hell about it. We'll make a spare door key to have in case the car washers lock the key in it but that's all. I do try to make sure that everything has a spare tire and a jack/tools, but most of the time that stuff is untouched. The story about taking the stuff out of another vehicle is probably true though. If we happened to be in a situation where a vehicle was missing something and the guy wanted THAT car, and he demanded the part, and I knew that part was somewhere else on the lot, I'd probably do it too. We are a smaller dealer though so that hasn't happened here.
IMO if it came with it off the transporter when it was new it should stay with the vehicle. I think to myself almost daily "if everyone would just stop stealing the crap it would stop being an issue almost immediately." Of course that won't happen but it'd be nice if it did.
Aftermarket accessories are a whole different critter though. If you leave a slide in receiver hitch/ball, or a trailer brake controller, etc... the detailer will probably cabbage onto it and out the door it will go without the front office ever knowing about it. 'Round here it will sit on a shelf for a month or so to ensure they don't come back in asking for it, but after that it's gone. I had a guy awhile back come in and ask for the change he left in the ashtray. Bastard stole my tip.
I guess it's too late to keep it short huh?