The agent I would recommend for mainland listings would be Bill Wildman. He's a Canadian who has been in Belize for 30 some years. He has surveyed land all over Belize, owns quite a bit of property in Belize and is, from everything I've been told and from my own observations, honest and dependable. He and his wife live near Corozal Town but he does a lot of business in Placencia. Wildman's business is Belize Lands Consultants, and they have a Web site.
Keep in mind that some of the agents you run into in Belize have no experience in real estate. There is no education or licensing requirement or any background checks. Many are expats who moved down and found selling real estate the easiest way to make a few bucks.
Also consider that most property in Belize is never listed with an agent -- you just have to ask around until you find something.
Adapter Kit: Belize
(the only comprehensive guide to living, retiring, buying property and investing in Belize)