The issue is, I believe, I just purchased this house and didn’t use his agency.

@Andrew ChungI've worked with these programs before and yes they do exist.Typically what happens is a developer received a grant from a Housing Finance Agency or Community Development Corporation (CDC) to build affordable homeownership units.

They have too many rules, crappy parking when rehabbing, uppity neighbors who complain and report you to the HOA and every government agency, you get fined when a 2 inch weed gets on your sidewalk, you have neighbors who play thumping music and you can hear them snoring and making weird noises when you are trying to sleep and a lot more.Don''t spend one penny you don't have to spend and move on to another project.

Your post is full of wrong info.Having done SEVERAL loans -- local bank (recourse) and agency (non recourse) -- including a $30m large refi, you're wrong. 

When I find a buyer should I get them to sign the Buyer Agency Agreement after they are pre-approved or on the first meeting?

Report them, by name, to every rental agency you can...and find another place to live...and when you get to your next place, report your medical problem to your new Landlord

Should I take him to small claims court or can/should I just send it to a collection agency?

Either though i reported them to several Federal law Enforcement Agencies, their just too stupid to stop!

As far as land is concerned, you can sell it for someone else to build a house on, donate it Habitat for Humanity, a church, or another agency(huge tax write off), eventually build a house on it myself(100k in construction could yield a $150k profit if the house sells for $250k...and you only paid a few hundred for the land plus another few hundred for the legal stuff lol), you can put a billboard on the land and rent it out, parking lot depending on the location, the options are truly limitless!

You could also check appfolio, that is what alot of the smaller agencies in my area run.