Honestly, since you’re the buyer, there’s no real reason not to use a realtor (because they’ll take their money from the seller, not you). I’d just be sure to get one who knows their stuff (i.e. – make sure they have some level of experience dealing with vacant land, as it’s a whole different animal than what most realtors are used to dealing with).
Market for Land is Less Active – The market for existing homes is almost always more vibrant than the land market. There simply are fewer numbers of buyers for vacant land than consumers looking for homes. Start marketing a new home listing and a new lot listing when both are desirable and priced well, and you generally can expect fewer contacts about the new lot listing.