The sign may be glitched. Just message the shop owner (snochabc) and tell him/her how it isn't working. They will replace the sign and hopefully it works
The only solution to this is that you re-log; which I am sure you've done. On the other hand, Snoch needs to re-place the sign on the chest so you can buy things; that will fix the problem.
Locking as inactive. If this is still an issue, create a new thread here: http://www.mineverse.com/forums/help.10/create-thread