Unused is a confusing term sometimes.

Non-stamp collecting meaning:
Used = worn from regular use.
Unused = New, not worn at all.
Mint = Really superb, excellent.
Stamp collecting meaning:
Used = stamp has been cancelled showing that it has gone through the mails.

Unused = stamp has missed being cancelled while going through the mails.

Mint = stamp that has not gone through the mails, post office fresh, hopefully with original gum, ready to be licked.

Since it is sometimes difficult to determine whether a particular stamp has gone through the mails or not

(it not being cancelled or otherwise 'used') some do soak them off the paper / envelopes and sell them on
ebay or elsewhere to be used again.
In Canada (and probably elsewhere also) it is against the laws of the land to reuse once-used or damaged postage stamps.

(I think in the US you actually have posters in your Post Offices warning against this. (We don't.)
Thus the brouhaha by Royal Mail (Great Britain),
Canada Post, the United States Postal Service and many others.

The Post Offices have already done the service paid for originally by the stamp(s) and now are doing double service when people are reusing used but uncancelled stamps.

The use of mint, gummed (hinged or not), undamaged, fresh stamps, however old, is considered OK and legal and not frowned upon at all.

I used to throw out uncancelled stamps when sorting through assortments as they were neither used nor new, to me. Now I keep them in case the stamp ends up having errors or varieties (colours, perfs, papers) that I didn't notice or didn't know about during the first sort through. Sometimes it's nice to have a copy of a stamp whether it is cancelled or not.