This is an updated version of the original post.
These are links to information and resources, suggested by posters here. These are categorized by subject. Also see the other pinned threads and the Blue Box sidebar for Half.com specific information.
This is currently heavily weighted towards Books. Please feel free to post informational links for Music, Movies and Games.

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GENERAL SITE RELATED
Link Your Half.com Shop to 'About Me Page eBay Page'.
Link
(Follow the 'Ready To Start' link.)
Half.com fee calculator:
Link
Finding Half.com sellers' listings
Link
Submit a Catalog Addition Request on Half.com
Link
Submit a Catalog Addition Request to MUZE:
Books (800) 935-4848 x346; E-MAIL, books@muze.com

Simple Explanation of Dropshippers, Spiderlisters, etal.
Link

POSTAGE & PACKAGING
Printing Postage via PAYPAL for non-ebay items
Link
ENDICIA Internet Postage lets you print postage for all your mail: Link
USPS
Shipping Assistant
Link
USPS Carrier Pickup for your packages: Link
Postage calculator
Link
Zip+4 data & display undeliverable addresses. Link
Track And Confirm! (Delivery Confirmation) USPS
Link
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How do you package cost effectively? Link
Weight Conversions
Link
Make boxes for your books. cost effective
Link
Archival & Related supplies: Link

BOOKS:
Bookselling 101: Link
Resources for booksellers & collectors: Link
Selling Books Online
Link
Easy and Affordable Solution to Operating an Online Bookstore: Link
free database/inventory software Homebase by ABE books.
Link
The Art of Books - Database By booksellers For booksellers
Link
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Price comparison sites:
Link
Link
Link
Link
Link
Link
Back-in-Print Bookstore
Link
Guide To Antique Book Values
Link
To check up to 10 books at a time on Amazon; shows selling prices & sales rank.
Link
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ISBN-13 Transition Support
Link
ISBN-13 Online Converter
Link
Frequently Asked Questions about the new ISBN standard
Link
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Information on Book Care and Repair
Link
How to clean soiled dust jackets/books? (Half.com thread)
Link
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Author signatures:
Link
Autograph Authentication Experts
Link
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Identifying Book Club Editions
Link
Link
Link
Link
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Info all about Nancy Drew books. Link
The old ones can't be listed on HDC (no ISBN)
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Independent Online Booksellers Association www.ioba.com85 IOBA Newsletter Link
Book terminology Link
Book-related terminology: Abebooks Glossary Link
Project Gutenberg online book catalog: Link
The Online Dictionary for Library and Information Science
Link
NY Times Best Seller Lists can be found at
Link

MOVIES
The internet movie database
Link
Disney link shows all DVD's and VHS
Link
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To report DVD bootleggers - e-mails.
Motion Picture Assoc of America, hotline@mpaa.org
Lawyers for Paramount, Dreamworks, Dreamworks Video and Universal Studios, dwilson@kmwlaw.com

MUSIC
CDs, Where you can listen to a snippet
Link
Music Information Searches
Link

GAMES
Check the informational threads authored by diskounters
Please post information references.

TRANSACTION PROBLEMS AND/OR SAFETY ISSUES
Feedback and transaction issues:
Here's the LINK to HDC's Buyer Protection Policy:
Link (Half.com thread)
eBay Feed Back Extortion
Link
Mutual Feedback Withdrawal for eBay/Half.com:
Link
Find negative and neutral feedback for buyers and sellers:
Link
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Evaluating/Reporting Fraud:
The FBI's Internet Crimes website. Link
Information on Internet fraud for the buyer and seller:
Link
Verifying a buyer:
Look up the buyer's name and address on Link
Link
Look at the map for the address to see if it is legit.
Link
Link or any other reverse phone directory is useful, as are Dun & Bradstreet & the other business databases.

Verifying a web address:
If you receive a suspicious email from a buyer, use the IP address from the email and look it up
Link
ARIN Amer. Reg. of Internet Numbers Link
The master registry for American & southern African internet addresses & maintains the Whois database of registrants for each domain name. Similar groups handle the rest of the world. A handy little program called NeoTrace will run a trace-back on any domain name or internet address in the world; sometimes you get the full Whois info; otherwise it will point you to where else to look.

INVENTORY RESOURCES
Find a thrift shop near you
Link
Find Auctions
to Link
Find book sales
Link
Link

MISC.
Making Suggestions to Improve eBay
Link
eBay search aids. (Not an eBay site.)
Link

Roman Numeral converter:
Link


And at the end of the day: How Not to Worry
Link
From the article: In The Path of Prayer (Viking, 2002; out-of-print, may be available from www.half.com),