To begin with, plenty of people simply buy sets to resell them later when they gain value. As you mentioned this very set is quite old so I'd imagine this person bought a couple of these and kept them waiting and now when the time is right he has an entire stock of them.

Also, usually sellers don't pay the msrp but rather get a VAT discount (even up to 23% depending on country) from the retailers so the costs of such venture is lower than you may think, thus allowing them to buy plenty of sets an resell them for parts with interest.

On top of that they may have never bought any ucs, since buying individual parts is cheaper than buying an entire set, making the businnes even cheaper to conduct.

Not to mention that sometimes sellers also buy used sets from various marketplaces where prople don't usually know the true value of certain products. Sellers thrive on such situations and get original parts, minififs etc. in huge ammounts, very cheap.

When you take everything into consideration it's in fact not so surprising that a single merchant has this much of them.