We're sorry to hear you've had a bad experience. We take this very seriously and have a process for resolving these issues.
Firstly, contact the seller by PM, clearly outlining your issues and stating what you want the seller to do to resolve the problem.
Then, if you do not get a proper response or an attempt to sort this out within a reasonable time (3 days is a good general rule), you should contact them and let them know you will take it further if they do not respond.
You are allowed to leave ONE negative review on the sales thread. You can be as detailed as you like, but it's important you stay factual and do not post personal information or abuse.
If you’ve already gone through this and the situation is still not resolved, then please look at submitting a thread in the Shitlist forum.
Here are more details on the procedure and rules.