UPDATE:
I was at my local Walmart today and decided to check out their current stock of portable DVD players, since this same question came up recently over on the PSR TUTORIAL forum. They had two models each from three different manufacturers. Of all six models in-stock, only one specified the external connections on the outside of the box, and those were for AUDIO/VIDEO OUT only - NO VIDEO IN. When these things first came out some years ago, almost all had VIDEO IN jacks. I think the idea was for optional portable large screen display of the hand held games like Gameboy. I guess with today's large screen tablet games, there is not much interest in this any more. I did find a couple of models on Amazon that specified a VIDEO IN jack, but they were up in the $70 (US) range, but still well below the typical Yamaha display replacement cost. Fortunately, the display on my PSR-3000 is still working fine, but my eyesight is not what it used to be, so I prefer a little larger screen. I have its VIDEO OUT connected to the VIDEO IN of an old 13 inch analog TV sitting on a bookshelf behind and just a little above the keyboard. The point is, the analog VIDEO IN of any old TV should work just fine. The portable DVD player recommendation is just for that - to maintain the PSR-3000's portability. For home based or studio based units, that never "travel", this should not be much of an issue.
Good luck !
Regards,
Ted