(Er... make that 3 parts! 'Cos AB won't let me do it in two!)
OK. Silly bit out of the way. One of the best ways to learn about the book trade is to study the excellent magazine, Book and Magazine Collector. (It's available in larger branches of W H Smith. If you're looking for it, note that the 'and Magazine' bit is only in small print. i.e. the title leaps out at you simply as 'Book Collector'). This publication is full of useful information and interesting articles; it's the standard reference for the seconhand and antiquarian book trade. It would probably be worth your while to get hold of some back issues. There are loads of these available on eBay. Buying single issues can work out rather expensive but this seller has got 45 copies on offer and is inviting an initial bid of �9.99:
http://cgi.ebay.co.uk/45-BOOK-AND-MAGAZINE-COL LECTOR-MAGAZINES_W0QQitemZ7021255330QQcategory Z1138QQrdZ1QQcmdZViewItem
The current edition of B&MC will include details of book fairs in your area. Go along and find out what seems to be selling and what prices are being charged. Also, speak to the organisers and find out how much they charge for a stand. (Many of the dealers at book fairs do it as a hobby. These events offer an excellent entry point into book dealing with a fairly minimal outlay). Book dealers, both in shops and at fairs, seem to fall into two distinct groups. They're either moody, sullen, grumpy and unhelpful or cheerful, chatty and happy to go out of there way to assist you (even if they won't get a sale from it). There seems to be no middle ground! Seek out some of the friendlier dealers and let them teach you the ins and outs of the trade.