I started on eBay buying books -- you know, those "oh, I remember those from childhood" favorites. Then I started selling. I still shop for vintage books on eBay, but a lot of auctions are missing vital information for a collector. Here's the information I like to see in a book auction:
Title - full title
Author or authors
Illustrator - this is KEY with children's books, as I often am looking for an illustrator (i.e. Eloise Wilkin, Corinne Malvern) more than an author
Publisher - be sure to also see the dustjacket and back of book, since Book Club Editions (i.e. Weekly Reader, Book of the Month Club, etc.) are frequently listed in these locations
Publication date - look at the copyright page and list the MOST RECENT date. I've made several purchases and been disappointed because auction seller listed the EARLIEST date instead of MOST RECENT, and I ended up with a reprint.
Type of book - Hardcover (with or without dustjacket?), trade paperback (larger paperback, i.e. same size as a hardcover book), mass market paperback (the smallest of the paperback sizes)
Edition - look at the copyright page again for the words FIRST EDITION or string of numbers, i.e. 10 9 8 7 6 5 4 3 2 1. I always include this with listings.
Description of book - What is this book about? This is also the place to interest illustrations that may be of interest to a collector - i.e. "lots of vintage illustrations featuring a cute Scottish Terrier" or "illustrations show travel on a Delta Airlines jet."
Condition - Be as specific as possible! Creases? Writing in book? Any dog-eared (turned down) pages? Spots or stains on pages? Foxing (tan coloration) on front or back endpapers? Wear at edges of dustjacket? Library markings? You can't list too much about condition!
I hope this is helpful ... I know this information helps me as a buyer, and I hope it helps my own buyers too!