So what should he do to computers he spent time and money to fix and clients abandoned? Throw them into the bin?
That's the whole problem. He tries for a couple of months to contact them (calls, emails, etc) and if they don't even respond, after around 9 months they get put into the resale bin.
His policy is "no fix, no fee" so I would presume there's no deposit. If there is no transfer of money it seems difficult to make even a weak legal argument that the laptops were "purchased". That is why Rossmann found the whole situation ridiculous.