• Catalog - Sellers can link* similar items (auctions and/or fixed price ads) to help buyers locate their grouped listings to save on shipping.   Examples:  Dolls, California, chromolith, etc. 
• Seller Designated Categories - Sellers can use this field to create their own categories or to flag items with strong keyword interest.  Sellers can then submit these 'keywords' for inclusion on the Category Keyword page by posting to the Category Forum.  Once the keyword is added to the Category Keyword page, buyers can browse or search this page to find the categories that are designated for each keyword, as well as an alphabetical list of all the keywords.
• Promotional Code - Sellers can group items for special offers such as "freeshipping" or "JanuaryClearance".  Promotions can be conditional on multiple purchases if clearly stated in the description field for each item included: "Search for my other January Clearance Catalog items using the Seller Catalog/Promotion field, and get an additional 50% off the shipping total for all January Clearance items shipped at the same time".
• Reference Number - Sellers can add their own item or reference numbers to help them quickly identify the storage location or any other designation they have for their items.
• Combination - combine any of the above.

Catalog or promotion groups should be linked in each of the seller's listings with a brief description such as "Search my Christmas Gift catalog" or "Save on shipping with my January Clearance items".  To make the link, use the Advanced Search form to create the search you want (search on your username and the keyword) and then copy the resulting URL as a hyperlink in your description.