When I book with a cheap cross-EU airline, such as Ryanair, Jet2 and Flybe, when I get close to departure day the pax is then expected to print their ticket, give their API and choose their seat. Often, for me, I'm not bothered about where I sit so don't pay the extra, so when I print my ticket there are just a few seats left to choose.
At some point, if the flight is "overbooked" won't that show up on the flight plan for someone trying to choose the seats when all have been chosen? Si if Ryanair sold 205 seats but only had capacity for 200, the last 5 will miss out even though they bought the flight?
Or do the budget airlines simply not overbook, so you pay peanuts but you get the seat?
And of course overbooking means passing your pax onto another flight, which may be overbooked and full, meaning pax will have to miss that flight etc etc.
Ryanair do not overbook. Almost all other airlines do.