Poloniex is not a wallet it is an exchange
And remember
If you don't own the private key of your Bitcoins - YOU DON'T OWN ANY BITCOINS´
If Poloniex will be hacked and your bitcoins will be stolen - you will not be able to do nothing.
My advise it to keep your bitcoins and other cryptos in a cold storage wallet such as Ledger nano ..or other cold storage wallet
RE: How Can I Buy Bitcoins?