I was thinking about making some things to sell on the market place but I want to ask you guys what is a reasonable amount of things to sell at once to the market place and for how much. I was thinking of making things like a crate, doors, things like that… maybe.
I have moved your thread to the correct location and added a prefix for you. 8-}
Some of your questions are answered here: https://forums.unrealengine/showthread.php?69402-Marketplace-FAQ-and-Guidelines
But if you would like to see what some people are asking for, try looking here: https://forums.unrealengine/showthread.php?25538-Marketplace-Content-What-would-you-like-to-see-from-the-community
Good luck. 8-}
It always depends on personal preferences + quality of the assets -> when I buy a door package for 15$ I personally would like to have a large amount of different doors in a good quality.
Just take a look at other marketplace packages that you can find and then decide the amount and price -> of course you can also use other business techniques (high price/low price strategy,…) 
If you’re going to do crates and doors, you need to do some really awesome crates and doors 
Those are two items that are pretty well represented already. The “Crazy Insane Door Pack” has an ahem insane number of doors, at a really cheap price.