In short, one word means objects people collect while the other means objects that can be collected–and is a variant of the first word.
Collectible means objects people collect. Comic books are collectibles. They are also collectable; able to be collected. Collectable is a variant of collectible, but some people still argue that it is properly used as defined above because some collectors collect things that have value only to them while a collectible has a monetary/market value.
The definition of collectible is able to be brought together into a series.
An example of collectible used as an adjective is the phrase collectible coins which means coins that people collect.
A collectible is defined as one of a group of objects that people collect.
An example of a collectible is the stuffed toy called a Beanie Baby.
Collectable–Variant of collectible.
that can be collected
suitable or desirable for collecting, as by a hobbyist
any of a class of old things, but not antiques, that people collect as a hobby, specif. a thing of no great intrinsic value
—yourdictionary.com The American Heritage® Dictionary of the English Language, 4th edition Copyright © 2010 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Webster’s New World College Dictionary Copyright © 2010 by Wiley Publishing, Inc., Cleveland, Ohio. Used by arrangement with John Wiley & Sons, Inc.