You can't buy whole poloniex for 1000 bitcoin for sure, check their daily trading volume

24hr Volume: 19852 BTC / 3617 ETH / 781 XMR / 278436 USDT

If you calculate 0.2% trading fee on those volume they are earning more than 40 btc everyday at minimum as today volume is one of the lowest trading volume per day i have seen. There are few days where volume rise upto 10 times than this on so basically one that single day they could earn 400 btc. So for sure they will not sell their whole platform for only 1000 btc.

NOTE: This are only my rough calculations not exact or verified one.