I don't know why it would be difficult to sell an apartment with a good tenant in place. Many investors are interested in rentals that are already producing an income.

The only reason it is easier to sell without a tenant, is that you can get the place all cleaned up spotless and it will stay tha way, and the agents can show it without giving notice.

You will miss out on buyers who intend to move in and occupy the unit themselves, but be realistic. How many people do you think are out there wanting to buy a studio for their own full time occupation? Not many; so you are not missing out on too many potential buyers.