I’ve never seen or heard any chatter about problematic Pinarello seat clamps before, unlike the notoriety of Campagnolo bolt failures. I wouldn’t let this stop me on what looks like might be a pretty nice frame.
If you’re buying over the internet, maybe you can ask the seller to send a pic of the removed bolt to a) get a better idea of how it works and 2) prove it can be removed without problems.
__________________
N = '96 Colnago C40, '04 Wilier Alpe D'Huez, '10 Colnago EPS, '85 Merckx Pro, '89 Merckx Century, '86 Tommasini Professional, '04 Teschner Aero FX Pro, '05 Alan Carbon Cross, '86 De Rosa Professional, '95 Gios Compact Pro, '95 Carrera Zeus, ‘81 Masi Gran Criterium, ‘81 Merckx Pro, ‘89 Cinelli Supercorsa, ‘83 Bianchi Specialissima, ‘VO Randonneur, Ritchey Breakaway Steel, Rivendell Rambouillet, Heron Randonneur, ‘92 Ciöcc Columbus EL