Is it easier to sell mono blocks or stereo amp on the used market?

Ask your dealer which is easier to sell. 

I think that you'll find that the price point not the form factor will be the determining factor.

Yup mono's more difficult to sell but please get what floats your boat. If we set up our systems looking for future resale values, then without a crystal ball it's likely moot anyway.

Also (Duhh) mono's need a pre that also may require selling. Then there's more power cords, shelf space, tubes and at lease twice a year dusting. It's exhausting to contemplate. 

I would select a stereo amp that can be bridged into mono, gives you or any future owner the ability to increase power should you/they need to.

read the small print of whatever you consider: some impedance taps are unavailable when you change to bridged mode.

@rick_n I agree with you.  Buying 2 mono block McIntosh amps at $7,500 each vs buying 2 Crown amps is the difference.  I have my mid-fi Emotiva monoblocks and they work for me whenever I want a 3 some :), and if I dont want them anymore.  Selling it for half the cost does just fine for me.