Categories I personally use more for organizing galleries than shops, but some people like to use them in shops just for the sake of organization (keeping CWs in a separate category, for instance, or expensive items that they might also be willing to trade, etc.)

The autopricer is a Gold Account feature that can be used to auto-price your entire shop at once. Super handy. :)

As far as tips for pricing, user shops aren't as fast-paced as you may be used to if you play on, say, Neopets. Most people stick with trying to match or beat the lowest usershop price, in my experience.