An assignment transfers your equitable interest in a purchase contract to a new buyer, who then closes with the seller in your place. Strictly speaking you're selling the contract, and your fee is the difference between the price in your contract and what the new buyer pays you for the position.
Whether doing that repeatedly requires a real estate license depends entirely on the state, and several states have added specific wholesaling provisions in recent years covering disclosure, how you may advertise, and how many transactions you can do in a year before licensing is triggered. Whether your particular activity falls inside those provisions is a legal question, so put it in front of a real estate attorney licensed where the parcel sits before you mail anything.
Two mechanical points that matter regardless of the licensing answer. Your contract has to actually be assignable, which means no clause prohibiting assignment and, in many forms, language naming you "and/or assigns" as buyer. And the seller generally needs to know an assignment may happen, which is both a fairness matter and, in some states, a written disclosure requirement.
The reason land has drawn less of this attention than houses is that the consumer-protection rules were mostly written around owner-occupants selling their homes. Land sellers are usually absentee owners and land buyers are often commercial, so the profile is different. That's a general pattern rather than an exemption, and it doesn't tell you what your state statute says.
