Most of these karts are offered by dealers, or by people who are getting discounted pricing for one reason or another. Obviously they have less into the kart than the retail customer. Additionally, they possibly rented the kart out, as described above, which gave them further equity in the kart. By selling it now, while its value is still quite high, relative to a new, retail priced kart, they can come out ahead, or at least break even.
Back when the exchange rate was more favorable for US customers, there was a much larger dealer margin and they could give their team drivers a new kart at every race and still sell the one race old chassis at a profit. That’s not as common anymore, but rentals help offset that.
Someone above said that the karts are trashed. That is almost never true. Although I’m sure there are cases when some clown wrecks the kart on the first weekend and decides to dump it, you would never see a reputable dealer do that. Often times the one race old karts are better than new, since they are already setup, sometimes have trick optional parts on them, and frequently are proven winners. These karts are often a great value. Remember, after your first weekend, your new kart will be one-race-old too.
One more thing to consider is that most (sponsored) team drivers’ karts are always for sale. If you know of a driver that is being provided karts and its the type of kart you want, go ask if its for sale. Sometimes you can get a great deal that way.