Also any transfer fees that may be applicable? 

For example, Tenant B doesn't pay their share of rent, so Tenant A pays the entire amount to avoid late fees or an eviction. now Tenant B is forced to go after Tenant B and you may not find out until months later.

I have a duplex in Echo Park and I am looking to install HVAC.  

I have a limit of two free range critters.If you do decide to accept that one, I hope they charge a hefty pet fee!

Can pay a flat fee listing service... 

A private installment note between two private parties (no banks). 

There is a big difference between a do it yourself handyman and a professional that does repairs and installations for a living day in and day out.

I have two buildings where the utilities are shared and to avoid high temperatures in the the apartment in the winter, I install programmable thermostats that limit the upper temperature.

Friend is willing to loan me the cash for full purchase price (for a fair fee).

Don't forget your closing costs on purchase and all your fee's when you sell.