It's a bit less opaque than that. Back in February or March, an email to Emiliano Sala from Willie McKay (the f*cking bastard of an agent who had tapped him up) was released, for reasons that are unclear to me.
Apart from being shockingly amateur (football agents write like 5th-grade kids, apparently), it detailed how McKay had rung up Sala's club (Nantes) and had offered to hawk Sala around to interested clubs to see what the price for him might be. Nantes apparently agreed, because they were curious about what his actual value on the market was, having received no offers for him up until then. And McKay had duly done the rounds at the lower mid-table clubs (Cardiff, Spam et al) and returned to Nantes with a credible offer from Cardiff (didn't say how much).
I suspect that route is one a lot of clubs use - the agent is an intermediary who finds out how much buying clubs are willing to pay for a certain player, and then returns to the selling club with that information.
So, a bit less of a shot in the dark, I would think.