As the others correctly say, you can sell it privately. No need to put it on the market.
You can either use a Realtor or a lawyer. If you use a Realtor, just be sure to negotiate the commission up front.
The first thing your tenants are likely to ask is: "How much?" So, before you approach them, you should have a defensible number in mind. It's generally pretty easy in condos to come up with good comps, and sales of identical or similar units (with the exception of short sales or foreclosures) all tend to be priced in the same narrow range. [I'd be glad to run the comps, if you'd like.]
If you have some equity in the condo, you might give them a bit of a break on the price. That's especially the case if you'll be saving some on the commission. It'll make the offer quite a bit more attractive to your tenants.
The second thing your tenants will worry about is that you might sell the condo out from under them. Your best strategy is to be honest with them. If your attitude is: If they want to buy it, great. Otherwise, I'll probably hold onto it--then tell them that. If your attitude is: I'm definitely going to sell it, but I would like to give them the first shot at it--again, be honest and tell them that.
Hope that helps.