One of the strategies that I am employing on DCity is to get and sell technology cards. It hasn't gone as planned with technology cards harder to come by than my initial calculations.
Anyway, one of the best ones that I have received so far is the Voice to Skull card. I put it on the market and it didn't move. Not surprising given the price. Recently, though, someone bought the card off me. My first success in the technology discovery approach.
I have another Free Education card which is not as valuable, but still worth a decent sum, on the market. If that were to be taken off my hands then that would be success story number two.
Planning to reinvest the proceeds in universities to maximize the drops of these tech cards although I am close to the max already. Would also need more workers given the current worker shortage that I am facing.