Blocktrades is an exchange, they have some affiliation with Steemit Inc, a place on their board i think. The Steem project was started on Blocktrades premises at first before Steemit got their own offices. Dlease doesn’t sell Steem or SP, you can rent SP from other users in P2P fashion from the market for a fraction of the cost of buying it, for as short as 1 week.