Why do you require identifying information to buy and sell bitcoin?

Aside from the typical KYC requirements, the most practical reason is that in the event of some problem with your transaction, we need to make sure we can correct it for you, not someone else. By getting just a bit of information from you before a transaction, we can both relax knowing that if something goes wrong mid-transaction (like a loss of network connectivity or a bill jam) we can make it right.