You often get the story from sellers that they come from main dealers who automatically change tyres on used cars for new ones. I have yet to see this take place.
A dealer will chance the tyres on a demo car before selling it, or a trade-in if it's going back on the forecourt, or a brand new car if the buyer specifies, or some new cars come on steels which are taken off and fitted with alloys that are shipped directly from the factory.
I used to work in a dealers and at one point when it was tyre/wheel clearout time, it took four or more LWB van loads to clear them out, and they went to local tyre fitter. Bet he loved that day.
As for people recommending budget tyres - I wish you would think before you type. Budget tyres don't grip as well, take longer to brake, and importantly perform very badly in the rain. And this is the UK.
They are referred to as 'ditchfinders' for a reason.
Add to that the fact that Alfas are generally owned by people who enjoy a more spirited drive (hence buying the Alfa, right?) and they need that grip. It's not a 998cc Daewoo we're talking about here.
Budget tyres are fitted by people who can't afford to maintain their car properly or who don't care about it. It is a false economy and dangerous to boot.
Part worns are a minefield too, most now come out of Germany with 3-4mm left on them, but check the inner wall for damage and check the date stamp on them so you don't get really old ones. Buying from a breaker is a good idea as you can get tyres that were perhaps recently on the road, maybe even fitted fairly recently before the car was crashed or whatever.
My BMW runs part-worn Bridgestones, my Alfa has brand new Falkens, my Skoda rally car has part-worn Pirellis on it and my Kawasaki has brand new Metzelers. my Fiat van has brand new budget tyres on it and it's had three or four of them in the last year.
Anyone who does any serious amount or intensity of driving will know that running the correct tyres is a MUST. And that a named brand does not the right tyre make. Michelin Eco tyres for example - best avoided at all costs.