They definitely all a challenge in their own way, replacing windows in sold blue stone walls is a treat, but the most frustrating issues are amateur rehab/repairs/maintenance over the many years that will haunt you.

I also get the sense that there's a lean towards "professionals" (with some notable exceptions as outlined by Jay) as holding fast and "amateurs" leaning towards discounts. 

This is because the 20% work their asses off to get the business (professionals) while the other 80% work at it when they want to work at it (amateurs).It is said that if you want something done, give it to a busy person.

So to me that is hefty price to be very much an amateur if you buy/sell one a year.   

I'm an amateur landlord in DC.

--> I can't speak to this with anything other than my amateur "gut" feeling -- you MIGHT be. 

We are complete amateur investors, but somehow managed to put all of our $75,000 into funds and four weeks later sell it all for $90,000. 

Is it a pro or an amateur?

I got a cold slap in the face when realize that for years I was just an amateur with a shotgun.

I suggest you skip any letter of intent, that says amateur all over it.