Gearbox seizures are most uncommon, TR, and if they do occur, it's unlikely you'd ride away immediately thereafter. Most likely it's engine seizure, but check the gearbox oil level to be sure.
Query the seller to see about the bike's history. A recent rebuild - say, within a thousand miles - could mean piston seizure. Drain the sump through a coffee filter paper and see if there is any debris, especially aluminum. If no debris, things are probably OK; you'll just need to ride another few hundred miles at 50 mph or less (some back roads are always pleasant, yes?), drain the oil, and add a dollop of STP or engine assembly lube to the fresh oil. Bear in mind that the GF is not quite as well-cooled at speed as its alloy head successors. But remember, oil is cheaper than any BSA part in the engine. Change it early and change it often.
As for the oil pressure release valve (OPV), see BSA Service Sheet 203. It's function is to protect the con.rod big ends. There is no published value for its release pressure. In any case, it serves to lubricate the camshaft after the rods are deemed to have been satisfied pressure-wise. If the OPV spring is missing, is too weak or the ball therein is held off its seat by swarf or other means, then the cam is getting preferential oiling and the rods are suffering. In either case, the pistons are are being "fling"oiled as long as the oil pump is performing its job, which seems to be the case since oil return is noticeable.
David