Upload & Sell: Off
Collections work within a single catalog. They refer to a subset of images (perhaps overlapping another collection) within the catalog. No catalog knows what is in other catalogs and you cannot open more than one catalog at a time. Better to use one catalog only for all images unless/until you hit a performance hurdle, but Lr 3 is far better than say Lr 1 in this regard.
Don't bother too much with collections at first. Use keywords instead. Start simple and add detail as required. You can easily restructure your keyword hierarchy at any time.
In Lr 4 you can have collections within collections, which may make them more useful.
Instead of using collections you might find it better to use colour labels and then select all images of each colour label in turn to do with them whatever you are trying to achieve (apply key words, relocate, etc.). Then remove the labels and go on to the next lot.
The combination of ratings, labels and keywords is very powerful but you also have a lot of metadata built into the images with exposure details, camera, date, etc. and you can sort or filter on much of that data.