You've got several options. You can build on them and monetize them by building content and traffic and adding ads either through affiliate networks or selling the ad space yourself. Or, you can build them up with content and traffic and then flip them. Sell the entire site.
Option 1 will generally garner you less as each time, but over a longer period of time and Option 2 will usually garner you a larger lump sum, but it's just the one time.
So really, you have to ask yourself if you want to maintain them and build revenue over time, or if you want to flip them and make a lump sum and leave it behind.