"Well, we'll mange," said Mrs Weasley, but she looked worried. "I expect we'll be able to pick up a lot of Ginny's things second-hand."
I think "pick up" here means "to buy". However, Ginny's things second-hand looks odd. Maybe, second-hand, as an adjective, is used as a complement? Otherwise, I think it might be better to put it as: pick up a lot of things second-hand for Ginny. Does it make any sense?