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

Stereo amps have a broader appeal.  Especially among novices and uninitiated.  “Yeah, cool.  What’s a mono?”

Amps and speakers in my experience are the hardest two things to resell. I have two of them in my current main system. OP, to your question though; I agree with the previous posters above that stereo amps are generally easier to resell. Because you're looking to buy Mac gear you really want to focus on responses from Mac only guys. The answers may be different from them. Good luck!

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.