I've been browsing 700c carbon rims on ebay, and noticed most are listed with the 33/50/60/88/etc mm in the title. What does this represent? It doesn't appear to be spoke/nipple holes.
Additionally, what's the general life span of a carbon rim?
That dimension refers to the depth of the rim. See this article on some particular carbon rims, that includes a cross-sectional diagram. Deeper wheels tend to be more aerodynamic, though they are heavier, and handle worse in crosswinds. Thus, different rim depths are more suitable for different types of races. 88mm rims are almost exclusively used in time trials and triathlons, while shallow rims will be more popular for races with lots of climbing. For riders without the luxury of multiple wheelsets, 38-60mm seems to be the standard depth.
The life span of carbon rims (or any rim, for that matter) depends greatly on the type of riding done. Typically, a wheel will need to be replaced because the braking surface has worn down. This means that the wheel's longevity is dependent on the type of riding done. Long descents with lots of braking will accelerate the wear. Riding in the rain will accelerate the wear. Keeping the brake pads and rims clean will increase the longevity. Also, some carbon wheels have carbon braking surfaces, which are more sensitive, while others have aluminum braking surfaces, which are more durable.
So I am hesitant to give an exact number, but I'd say a reasonably-well maintained wheelset should probably last 10,000-20,000 miles.