current buys on bitstamp:
560 66
555 321
550 1239
545 1801
540 2611
535 2852
530 3103
525 3539
520 3974
515 4127
510 4334
505 4557
500 4970
495 5265
490 5418

and there you come with your 30000.
It will cause a drop from 560 to about 500 if they would be sold in one time. Ridiculous.
2 Minutes later, the price would be at least at 540 again.