In Kürze: QSL am liebsten über das Büro, direkt ist aber auch in Ordnung.
I am an active collector of QSL cards, so I am always happy to receive yours! I prefer QSL via bureau, but if that’s too complicated or impossible for you, I also appreciate direct QSL. IRCs or „green stamps“ are not required, but a self-addressed envelope would be nice. It goes without saying that valid cards are always replied. You can find my address here.
I keep a record of QSL cards sent, primarily for myself so I will not send out cards twice by mistake. It is only sorted by the month I sent out the card, but it might still prove helpful for you, so I might as well share it publicly. Also I can not assure that there are no call signs missing that I sent out cards to, but if you’re in the list a QSL has definitely been sent, usually via the bureau.
A record of QSL cards received might be added some day, as well as an online log of some sort.
While I do have an eQSL account, I don’t actively use that service – it’s „paper only“ for me.