MP4 players market has been never exploded because there are a LOT (and really a LOT) of items like smartphones, pda, that play MP4 and do LOT of other things.
Besides watching a movie on a 1,8" monitor would be really really annoying.
MP4Players with bigger monitors cost exactly or maybe more 2,8" touchscreen pda at today.
But however these domains represent a niche, and i would say they have some value.
Best / MP4 / Players /com around mid $xx
MP4 / Price /com high $x - low $xx
MP4 / Prices /com mid-high $xx.
Last one has good development potential, maybe a basic eBay/Amazon website with commissions and some ADSense.