They buy it privately people, if you actually see 18 k buy I would be amazed..
You figure they know someone who has 18k on hand who doesn't want to keep 18k on hand?
I'm not saying no one will do a private 18k sale, but that sort of person will probably be passing that demand right back into markets you can see.
there should be quite a few people owning >= 18 kBTC. in 2010 there where probably no more than 1000 people mining. let 20% (1000 * 0.2 = 200) of them have mined 80% of the coins (5 mio * 0.8 = 4 mio), then there's 200 people owning on average 20 kBTC. Do that pareto-trick again and you have 40 people owning 80 kBTC each
, 8 people with 256 kBTC and 1 or 2 with over a million.
I know that's very rough guess
timating here, but it gives you an idea: Bitcoins are not evenly distributed and some people might even shell out 18k for some fiat without subsequently re-buying it on the market... they might even be waiting for a buyer like that because they might need fiat and don't want to hurt the exchange rate.