It's always nice when you can sell cobblestone/stone, dirt. They are useless after a while and some people (like me) feel like they still need to collect it. When you can sell it for some cash (even if not much), that's a good thing.
Although I think that this can't go for loots like Zombie flesh as you can AFK farm a lot of it and if you make it too cheap to sell, so it won't worth it to the player, the trade won't be used so it wouldn't make any sense.
I also think adding precious items (diamond, emerald, beacon, elytra) can be good too. They can set a general price in the community, so players can sell it for at least a bit higher to players.
Anything inbetween is nice too, so food items, etc, but they are not as interesting.
What I would make worthwhile is wood maybe. That feels like a job (like the jobs addon) and selling it should be somewhat rewarding (definately more than cobblestone).
I hope I could say something useful :)
