Okay, you have a great item, and you've written an excellent description and a compelling subject line. Now it's time to post the item. Which category should you use? That ought to be obvious, and it is. But the obvious choice is not always best. It is sometimes necessary to think beyond the obvious to reach the largest number of potential buyers, unless you want to give someone a great deal. (Some of my best buys at online auctions were the result of someone posting in a category where no one saw the items. I'm creative in searching, though, so I was able to find what everyone else missed. I'm not the only creative searcher out there.)
As an example, imagine that you want to sell a 1961 issue of a magazine called Mechanix Illustrated. The obvious place to post this would be in a "Magazines/Back Issues" category.
But wait a minute! What sorts of articles are in the magazine? Is there anything of special appeal? Say it's the March 1961 issue of Mechanix Illustrated. This has a rare interview with a young rookie NASCAR driver named Richard Petty. Wow-that's one for the race fans. So, forget the "Magazines/Back Issues" category, and head for a category that covers NASCAR (probably something like "Sports/Racing/NASCAR.") This is where NASCAR fans are most likely to be browsing. But whether they're browsing or searching, NASCAR fans who spot this item in the "Magazines/Back Issues" category might just give it a miss. If it's in the category that matches their interest, they'll be sure to take a look.
Similarly, an original sales brochure for a rare automobile doesn't necessarily belong in an Automotive category. Rather, you'd put it in a Collectibles category for Automobilia. Why? It's not an owner's manual, nor is it exactly an accessory. But it is collectible. Further, even those who don't own the automobile in question may still seek such an item in a Collectibles category.
A vintage 1920s radio? Forget "Consumer Electronics." Buyers there want new stuff. Dig around until you find the appropriate category in "Collectibles," or maybe even "Antiques."
What about posting in two categories? Well, if your item has the potential to bring enough to make it worthwhile (don't forget the extra cost), and attract more bidders across both categories, go for it. Post that 1939 Crosley sales brochure in Collectibles and in an appropriate Automotive category.
You might do the same with a 1930s Tootsietoy catalog. Post it in a "Catalogs" category, but don't forget "Toys/Tootsietoy." Or, back to Crosley autos, how about that February 17, 1947, issue of Life magazine with the profile of Powel Crosley, Jr. and his line of compact cars? Forget "Magazines/Back Issues," and go directly the Automotive category that is appropriate for Crosleys. But, again, don't ignore the collectors: Post the magazine in the "Collectibles/Automobilia" category, too.
The bottom line? It's your job to put your items in front of those who are most likely to buy. To do that, you have to think like the buyers. (And remember this: Not everyone thinks like you.)
Of course, it doesn't hurt to do some research into the categories used by those selling similar items. Do keyword and phrase searches in all categories at an auction site for items like yours. If the auction site permits, sort the search by price, so you can find out which categories have brought the best prices. And don't limit the search to active items; searching closed auctions will turn up the most useful information.
Finally, make sure you don't select a category that's totally irrelevant. Auction site management or self-appointed "Topic Cops" can get your auction cancelled if it's too far out of category.