What to do if I signed contract stating that when my wedding dress was sold I'd be notified and paid but the dress has been sold yet I have not seen a check?

You could sue the other party for breach of contract (not doing what they agreed contractually to do), since contracts are legally enforceable. For amounts up to, say, $1,500 or $2,000, you should probably sue in small claims court, acting as your own attorney; for larger amounts, you may wish to hire a lawyer.