hi, i'm new here. where is a good platform for me to buy and sell bitcoin when it go up and down? thank you.
Welcome. We have a sub here "Trading Discussions" in the "Economy" section. Probably best to move the thread there. Also this depends on what you want to trade. Altcoins or other currencies. There are really more exchanges then i can list here. But here are a few:
bittrex
yobit
bter
coinexchange
hitbtc
bleutrade
cryptopia
Just take a look and see for yourself what you like.