Any Belizean owner who lists property with a real estate company probably has an inflated view of its value and is looking for a sucker ... er, I mean, a foreigner, to buy.
If you really want to buy property in Belize, spend a few weeks driving or busing around the country. When you see a piece of property you like, start asking around locally to see who owns it or who else nearby has a similar property. Then start talking cash. Or better still, find a local resident who will make the offer for you.
(This usually won't work on Ambergris Caye, though if you look hard enough at property that's been for sale for years it may work here, too.)