Functional items are neither more nor less desirable than nonfunctional items in themselves. Among glass and ceramics collectibles, in fact, it is often the decorative wares (vases, cachepots, figures) rather than the functional tablewares (plates and cups and saucers) that are most highly sought after. Collectors generally prefer their furniture to be functional, however. There's not an active market in unsittable chairs.
If the object does have a function, it should be in working order and retain all its parts. While it may not be working at the time you're thinking about buying it (many old watches, for example, are not), if all the parts are there, it may still be repairable.
The wonderful thing about twentieth-century collectibles is their frequent and reassuring labeling. Yet labels and makers' marks don't necessarily guarantee quality, and they can also be faked, so look at the entire piece carefully. Attribution to a particular maker is most convincingly made on the basis of a label, but the piece itself should support the attribution. Some experts, in fact, think that the label is the last thing you should look at. When there is no label, scholars and collectors can often make an attribution based on the object's stylistic and material resemblance to known, labeled pieces. Despite the possibility of eventually making an accurate attribution, however, almost every collector prefers to own an item that retains its original label to owning the identical item, unlabeled.
History of Ownership
History of ownership is among the least reliable methods of evaluation for twentieth-century objects, but luckily, it is usually one of the least important ones, as well. Consequently, it appears fairly low on this list. Twentieth-century objects haven't had time to accumulate much of a history. Despite the wonderful things that have been saved and brought into the Antique Roadshow, a great deal of the past century was more about disposability than about preservation. Many Americans no longer saved their material culture to the extent that their ancestors did. Thus, the history of ownership of many objects is often no more than anecdote, and anecdote is almost always dubious. It is altogether a different horse from the far more rigorous history of ownership known as provenance, which is a very important part of evaluating antiques and works of art. A painting by Monet with a complete, documented history of ownership from the time it left the artist's studio is much less likely to be a fake than one that has mysteriously appeared on the market for the first time. An eighteenth-century desk from Newport is more likely to be authentic if it has been passed down in one Rhode Island family for the last two hundred years.