Other landlords charge a flat fee, ranging from $50 to $500, depending on the market. 

Take a walk through of the apartments this weekend and see if the apartments need a lot of repair. if this is the case - bake this into your calculation.Good luck!

During this time, the markets have been pretty favorable and probably have covered up mistakes that would eliminate (or created negative) returns in a down or flat market.Really depends on the investor.

It is a 2 flat with a 3/2 and a 2/1.

I have several other properties that are two flats, SFHs, condos etc. some of which have 30 year fixed and some of these are paid off. 

There have been a few discussions in the past regarding small flat-annual fee and transaction-fee only brokers out there, but I'm thinking through a scenario where, I, as the investor using my own cash, I wouldn't be paying commissions to myself, which means I would completely dodge the transaction fees and would only be subject to paying the annual fee of around $200.

What if a site guaranteed a certain number of inquiries per year for a flat rate model?

In addition to the commission that is technically owed by you, but in practice is almost always offset by the listing agent cooperation fee (usually 2.5-3.0%, at least in my area), there is sometimes a flat fee or admin fee changed by the buyer broker.  

grandpa lived there.. still stuff and vehicles on property... needs work, but its a BIG house, with a granny flat added on upstairs... 

The lot is clear and flat already.