Avoid any broker who wants any money up front.
Avoid any broker who wants you to sign over your deed up front.
Use a reputable closing service so buyer and seller have an independent escrow to process the transactions.
Check the BBB and state licensing agency to see if the broker's business record is in good order and if their broker license is in good standing with no disciplinary actions or complaints.
Talk with the folks at The Timeshare Store (our forum sponsor -- see top of page). I have used them for multiple sales and purchases and very much appreciate their advice and professionalism.