how can i ...

build trust up whenever i post to sell account i get called a scammer just because im a leecher

 

Honestly, just get more posts. You might get lucky and find a buyer, which would boost you, but really until you accumulate a high number of posts (in the hundreds) people are going to be very reluctant to buy from you.

 

Sadly thats mostly true with that black market.
Maybe try to do user services to get vouches?

 

When I first sold an account, I was a leecher and people didn't seem to be bothered about it.

However, if you get scammed, people use it against you. So make sure you always use a middleman, and make a secure trade.