My wife and I recently purchased a 3 flat in Chicago.

Those residents pay a flat fee based on a few factors (i.e. number of fixtures, square footage, etc.).I assume you did your research on what you could rent this for...As is, I would not pursue this deal unless you are banking on appreciation and being able to increase the rent quickly. 

The flat I’m buying is in Rubí (close to Barcelona), 128k, 3 bedrooms, 1 bath, renting long term for 600-750€.

Also, you don’t have “use a % to avoid net listings”, you can certainly be paid, and offer to the buyer agent, a “flat fee”....has nothing to do with being a net listing.

Landlording can flat wear you out at times. 

Stay away from flat or declining neighbourhoods. 2.

(Full disclosure, I just plucked this off Loopnet so likely the numbers are baked more than the crowd at a Dave concert)  http://www.loopnet.com/for-sale/omaha-ne/multifamily/?

There are good flat cabinet investor friendly companies out there.

Has anyone had success with a flat rate early termination fee in the past?

@Alan J Mowder Don't forget about the middle ground between FSBO and Agent - that being "Flat Fee MLS", where you get the benefits of listing on the MLS (exposure, offer commission to buyer agents, etc) without the big listing fee that comes with a traditional agent.