Wholesaling Properties on the MLS

[Kevin Green]

In regards to wholesaling, when a purchase agreement is signed between the seller and wholesaler for a property that is on the MLS, what's to stop the seller from turning around and signing with another buyer? Because technically, since I would not be going through an agent, there is no way for me to mark the property as "under contract" on the MLS. How do other wholesalers deal with this? What steps can I take to prevent this?