[LFCooledWhip]

I want to buy something in the marketplace and make a post to buy something but I can't because I have neg rep. What do I do?

[Trappy]

You will have to earn positive reputation, you will be able to post in the marketplace once your total rep is 0.

[/wrists]

You should cry a little harder.

The majority of -reps on my profile is all dumb children who give -reps to people who don't have the same opinion as me.

[Saltpeter]

You contribute to the community and earn positive reputation. Threads like this are how you earn negative.

[Mαxim]

I think you know the answer to your own question but i'll play along. Reputation is earned, not given. You can read more about the reputation system Popularity System.