[mothered]

As far as I understood the method, after the item has been placed back into stock and then sent out to another customer, they can't trace who did what and when. Is this assumption wrong?

That Is correct.

Given It's placed back Into stock, then picked, packed and dispatched and transported by the carrier until It reaches Its destination (the buyer), It goes through so many hands. As such, It cannot be pinpointed to yourself - the social engineer.

My concern now is that I brought the parcel to the post office last Saturday and they received it on Tuesday. Usually, I would get a refund confirmation mail one or two days later. I'm now assuming that they actually wait until they checked the return as it's a high value item. Is that timespan normal for high value items?

Refund confirmations are not always consistent.

Under similar circumstances, I've experienced a couple that took 2 weeks to finalize. It won't hurt to contact them In a few days or so, asking for an ETA on the refund.