They are basically selling real estate without a license and making contracts to buy properties they don't have the means to close on. Usually they don't disclose this. I have bought one property in Mississippi through a wholesaler. It was a property that could not be financed because it had a second unpermitted house on the land. Mississippi doesn't much care about such matters but lenders do. In this case the original owner, the wholesaler and myself all knew what was going on, so there was no deception involved.
There was another property in Mississippi I was trying to buy that was listed on MLS but someone made an offer that was accepted before I could make one. Soon afterwards a guy was trying to wholesale it on Facebook (not legal, btw.) Usually when wholesalers can't find a buyer before the close, they will try and get an extension. Knowing this I had my real estate agent contact the seller's agent, let them know the situation and then made a back-up offer asking for only 15 days to close on a cash deal. When the wholesaler failed to close, I got the property.