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

Both are popular in this hobby. It totally depends on the model in question. 

Rogue Audio claims they broke up the Zeus 225 lbs stereo tube monster into Apollo monoblocks because of the logistics of moving the damn thing. When you have a single chassis weighing beyond 100 lbs, it gets pretty difficult without hiring help - especially secondary market sales w/ shipping. Monoblocks help break that up, though then you're paying for shipping 2x boxes, 2x power cords, 2x amp stands...

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.