My next main suggestion is Warp Signs.
No /warp command, just signs. Means we could have a spawn building on the Factions world with signs that can warp you to the Market, even install a premium market with higher selling prices.
Also makes it easier to control who can go to what /warps in a way, but then again with factions there is the teleport command so things like that would be hard to control, people could easily teleport other players into premium zones so... :/
The pricing I had thought of: Regular Markets would be something like $32 to sell regular stacks, like wood, cobble etc.
Then to buy those regular stacks, you would pay either $96 or $128. Then make all the other items scale in line with that or something.
Just an idea, that way it wouldn't be massively easy to make money, and it gives you something to work towards.
Then in premium markets, sell price could be $64 and buy price could be either the same or slightly lower, but not massively lower.
As for the regular world, yeah, the idea would be to make it like SSP but on a Multiplayer world, so it has the added challenge of other players.
Then you have the option of trusting certain players to be able to come into your house with that plugin, or just play alone and avoid contact.
I think there are ways to disable plugins on certain worlds, so only have the grief protection plugin on that world. (The one where you need a gold shovel and gold blocks to protect your house)
It could almost be like the spawn world would be a hub, and then you can teleport to different "severs" that all act different and are all separate from each other (money/items don't cross over etc.) but as I said before this kind of stuff should be for further down the road, not initial set up.
Just a factions world and a hub world sounds good to me right now.
Anyway, all of what I'm saying is just ideas, you can mould them into whatever you want.
As a side note, as I have always said:
If you guys are building, message me on Steam.