I love searching for valuable collectibles and getting excited when an auction ends high for me.  But I have tanked out plenty of times.  Although I am still learning, these basic guidelines have helped me out.

Do a completed search on Ebay with the items that are the highest price first in a selected category.  Check out whats selling and has a lot of bids.

What to look for:

Books:  In general, 1st Editions, with dustjackets.  Books on collecting, ID guides, and old cookbooks usually are a pretty safe buy.  Many garage sales and thrift shops sell books for as little as 25 cents a piece!

Dolls and Toys:  With Modern dolls, some Company names to look out for are: Berenguer, Ashton Drake, Madame Alexander,etc.  Old Barbie dolls are usually a good bet. The older the better.  1960 / 1950's dolls  like Chatty Cathy and Dancerella.  One thing with older dolls that happens for me is, the uglier the doll, the better!  lol

Toys:  Again, the older the better.  Old wooden toys and the like. 

Dishes / Glassware:  Soup Tureens w/ lids,  Large Pyrex mixing bowls, Vintage coffee cups with company names like Dunkin Donuts and Texaco, Fire King Jadite, and some older juice glasses sets are valuable because they are not marked and must be identified by pattern.

Carnival Glass:   Some colors and patterns are more rare than others.  Patterns like acorns, cherries, and peacocks usually sell good.  Good Luck bowls sell good.  There are many reproductions and carnival glass takes some time to identify correctly. 

Pottery:    Look for company marks like Stangl, Roseville, Rookwood, Van Briggle, Newcomb College, and Belleek.

Glass:  General note on old glass.  Old glass has a rough circle hole (pontil) on the bottom and is more imperfect looking from handblowing.  Ball Jars can be valuable if they are deep in color and especially if  "Ball"  is upside down instead of right side up.  Old druggist Apothecary jars and sets. 

General:  Old iron stuff like door latches, locks, and door knockers.  Old antique hand tools,  old glass dresser knobs (colored is a bonus for me).

Bottom line:  Do your homework.  There are many great resources and links on the Ebay discussion boards for identifying your collectibles. 

Don't spend a lot of money unless your sure!  Stick to your hunches.