Only Operators can use the sell and buy signs but obviously, that is problematic. I get no errors and I am certain my permissions are correct.

Perms: http://hastebin.com/vadisifila.avrasm

My spawn protection is 0, btw.

Please read the correct documentation: http://ess.khhq.net/wiki/Command_Reference/Perm
Why would you think that your permissions are certainly right, when they are clearly not?

I just realised that while I was testing now. I found online that those were the "correct" permissions. Apparently not. Thanks for your help tho.