Hi Newseller,
Please note that while the LOI is not really that binding, the sales commission agreement can be. Check the document with a qualified advisor.
Sellers are not supposed to sign anything before prospects are shown, and when that happens the legal documentation is a signed \"offer to purchase\" by the Buyer. These documents that he presented to you are dodgy with regards to real estate business and are not how sales are supposed to be done. I suspect this is NOT a real estate agent but someone posing as one to cut himself a piece of the action. The selling price fixing along with you declaring that anything above a certain price is his commission further indicates that he is acting without Buyer's knowledge or consent. There is a possibility that his entire story about the 3% agreed to by the Buyer is an outright lie, perhaps there is no Buyer in the first place.
If you have his card and he indeed has an E-number, you should definitely report him to the real estate board.
If you are however very interested in finding that Buyer if the price is really good, then you can call the agency and real estate board to check on the agent, then ask to meet the prospect. After securing the contact you can then insist on paying only the legal fee listed by the board or else you will report him.
My own opinion though: don't sign the papers and report him. If there really is a Buyer then your land is worth than what you will get through this agent, and if you deal with him you might be party to something devious.