Coops that allow subletting only allow a certain percentage (usually less than 10%) of units to be sublet. Some Coops don't allow subletting at all. Others allow it only with a proven hardship. I believe the lenders refuse to lend to Coops with greater than 10% of sublets in occupancy.
From a traditional standpoint, I hate the idea of Coops as an investment. Maintenance fees suck, and I absolutely hate the idea of a group of people meeting every month to rule their land. Unlike a fortune 500 company that is run by a Board of experts, all you need in a Coop is a buy-in. So, you can have people with no sense of business on a Board just because they like to be in everyone's personal business.
If, however, you can find a way to bypass a Board where their rules are limited on your actions, you stack the odds a little more in your favor. If you can legally rent without them approving the renter or the idea of renting, you are doing so around an environment set forth with services like live-in Superintendent, exterior and common area maintenance, heat, water, sewer and taxes covered in your maintenance payments. Sure you're going to pay $600-$1000+ a month in upkeep, but it's like keeping money aside for capex, repairs and taxes.
I lived in my parents' Coop from 8 to 22 years old growing up, and when I turned 22 I began managing Coop buildings full time. Having been involved with them my whole life, I am fully dissuaded from ever LIVING in one again, but as I work out my own business plan, I see potential if you can find a way to invest them legally.
I can't say how hard it is to sell a Coop since I haven't really focused on it. Since Westchester property taxes were the highest in the US last year, Coops in this area are a more attractive option for first time home buyers or retired or those that don't want to maintain their exterior etc etc. Therefore I'd like to think they're relatively liquid. According to one of my building's comps in White Plains, what used to sell for $100K-$130K in 1998 now sells for $300K-$325K. That's nearly a $10K increase in value per year over 22 years. And at least 1 unit of that size has sold on an average of 1 per year or more.