From what I can see, most of the ones for sale have probably come from stamps collected for charity - dealer pays a small amount per kilo, sorts through them, picks out those that have missed the franking machine, soaks them off, dries them and sells them on e-bay. Most are honest enough to say 'ungummed'.
It's YOU who are committing an offence if you use them - he's 'just selling to a collector, guv'. On the other hand, undetectible if a bit fiddly gluing them on envelopes to send.
Few businesses buy stamps these days - most use franking machines (and get a discount on postage for doing so). Those that do legitimately buy stamps can sell them back to the Royal Mail for a small commission and are unlikely to sell them elsewhere at a discount.
Anyone selling large quantities (eg sheets or books) of really unused current stamps on e-bay has probably nicked them - or got them from someone who has.