Why do listings say salt or candle box?

Because the two are the same object. A small lidded wall box near the hearth suited either salt or tapers, and nothing in the surviving form settles which it held. Dealers reflect that. The word "candle" sits in 15 of the 54 titles here, and several read "salt or candle box" on a single piece.

What wood are they usually made from?

Oak, more often than anything else: 24 of 54 titles name it. Mahogany appears in several listings, including a George III and a 19th-century example. Pine, elm and beech each turn up once or twice, and one box is Welsh pine.

How many come up for sale each year?

About four a year on Sellingantiques. Thirty-four different dealers have listed one, which works out at roughly one apiece. No dealer treats them as a stock line, so they arrive singly rather than in runs.

Are any of these boxes marked by a maker?

No. These were kitchen boxes made without signature, and the listings return no maker names at all. What listings name instead is timber, period band and sometimes a region.

Which regions are named in the descriptions?

Regional attributions are scattered and few. Scottish appears on three listings, Welsh on two, and single listings claim French, Swedish and generic British origin. The Swedish example is described as being in original paint.

How reliable are the dates given?

Read them as shorthand. The date field is mostly a band: 19th Century, 18th Century, Georgian, 1800s, 17th Century. Only eight listings carry a clean year, and those run from 1800 to 1870. The bands are also used loosely, with a "Georgian oak salt box" sitting in the 17th Century field.