I'm not buying the "it's commonly used, it's not an error" argument. Grocers have had a rather famous tendency to use random apostrophes, that doesn't mean that anyone copying them is somehow correct. I know that "off" is used quite deliberately in certain situations (engineering companies and some stock control scenarios), but that still doesn't mean that it's not born out of an original error. I still don't believe that all the sellers on ebay that use "off" are doing so for any specific reason, or that they wouldn't be better off using "of".
I'm not really selling it.
There's no reason not to say that whilst "
n off" has a history of legitimate use in specific work contexts, it's being used out of context on eBay, perhaps by people thinking it's the right or professional-sounding way to quantify items. Perhaps it's uncommon enough now to draw attention to itself and look weird.
But such changes are always on the go, and in the end, usage is king, and if enough people use a word in an altered sense or new context, it ceases to be a "mistake".
Words broaden their range - a holiday was a HOLY DAY; now it's any time off or a trip abroad etc
They narrow it - a girl was a child of either sex in the middle ages. (No kidding)
They pick up better meanings - nice used to mean sly and sneaky
They pick up worse meanings - cowboys were the good guys in the film world of the post-war period; now they bodge your building repairs.
Imbecile, idiot and moron were once technical terms denoting IQ levels in the language of primitive psychology.
The range of meanings and behaviour of words has always been more fluid than is popularly thought. People respond to this in different ways. For some, language is what people do; for others, what they ought to do. Most appreciate, intuitively, the tension between those poles. People come up with all kinds of new uses; others fight rearguard actions against them. It's a jungle out there.
Spelling/punctuation - the apostrophe business, eg - is a very different matter, as it's relatively stable and limited, and a sphere where you can say that's a mistake. Equally, if enough people stop bothering to use apostrophes on the basis that they're confused and don't care, it could fade out of use. There is a society dedicated to preserving the apostrophe.
The world of vocabulary meaning and use is much less clear-cut and far more open to variety and change.
Excuse the long reply - I'm debating whether to go fishing or not, and hanging around the laptop.
