Here is a short list with a few things I think should be added to the market;
slimeballs (people are ALWAYS asking on chat and no one has them)
pistons and sticky pistons (this seems like a must have all round)
redstone repeaters
And there is probably more but cbb right now, please comment on your views for the stuff I said or suggest something else to be added?
I just think the market needs to be expanded a bit as it only has a few things out of the kajillion minecraft items <3
Slimeballs: They are rare, that's why you don't get many people selling them. I, myself, have 18 of them. Slimes aren't impossible to find, but do require effort and knowledge. Go into a Slime fight with the right weapons to yield the highest slimeball drops (yes different weapons change your drop chance).
Pistons: Really? These are so easy to make. I made over 50 of them in the span of a week without even trying. The materials are available on the market if you can't be bothered to mine.
Sticky Pistons: See both answers above.
Redstone Repeaters: Don't be lazy, these aren't that hard to make either. Stone, sticks, & redstone is all you need. As with pistons, if you can't be bothered to mine all of the components are on the market already.