The only issue with that is with your end buyer. You would have to get the property below the listed value by a good amount in order for you to be able to add your fee and still sell it to an end buyer.
Every good buyer on there is constantly keeping an eye on the MLS for there area. If the house is listed, chances are many investors have seen it and if it hasnt sold it means they passed on the deal. For a realtor to even be talking to you knowing you are a wholesaler, it means that they have not been able to sell it directly to an end buyer. In that case you must have excellent (and i mean excellent) negotiation skills in order to secure a contract way below listing price.
If you secure a contract at listing price (very doubtful) you would most likely not get a buyer because chances are that was already a bad deal for investors in the first place. You would have to add your fee to the listing price. As an end buyer i would not be happy if you sent me a deal that i find out is on the MLS for a cheaper price.
This is why its not the best idea to wholesale MLS properties.