I personally don't sell my old cell phones or computers, I don't even throw them in the trash - not only to prevent anyone from getting any data, but also because one day all these old devices will be a good reminder of how technology has evolved over time. In case someone still wants to sell an old computer, I wouldn't risk deleting the files - just take out the HDD/SSD and sell it that way.
I do, I've successfully recovered both wallets (Electrum and Metamask) and they are up and running on another computer. Even if data loss is presented, both wallets are fine, however, my only concern would be the technician to snoop through my files, which may sound too paranoid (or not, you never know).
I would just in case transfer everything to new wallets, no matter how much you believe or not that person who did the diagnostics on your computer. Although we all think that such people keep their reputation and will not do anything bad that would endanger it, sometimes it is hard to resist not peeking into other people's secrets if you already have the opportunity.