Also, capital gains is not calculated on what you receive at the closing table, but what your sale price is less the book value on your schedule E (?). 

I am trying to get my next one but having issues on the 4th bringing $ to the table.  

The other day I was at the closing table waiting for our lender to send the CD to finalize everything.

That’s something that will always be on the table in urban markets.

It may not be fun 'thinking' you'll be leaving money on the table by lowering your rent... but really... lowering you rent by a mere $25-$50 can be enough to make your rent irresistible and you'll likely recoup that in decreased vacancy. 

However, you have to bring significant value to the table.  

I think you have a few good options on the table.

What percentage of a deal is fair for me to ask for when I bring a good cash flowing deal to the table?

If your motivation is top market value, then I recommend you visit with a few experienced local Realtors to ensure you aren't leaving a ton of equity on the table if you do sell directly to a company. 

Its never a bad idea to take some money off the table.