Why did Winston buy the diary? Where did he buy it from? why was this a dangerous thing to do?

Winston initially bought the diary because it was old and looked beautiful. He bought it from the Prole section from Mr. Charrington's antique shop. There were no laws against doing anything. Unorthodox practices, like buying and writing in a diary, were seen as a grave sickness in the mind and people were usually vaporized (killed). Their records were erased and the Party made it so these people never existed in the first place.

why did Winston write in the diary