I've bought a modern car from an auction - albeit it was 30 years ago nearly - it was a modern car then (1987) - but not bought a classic from one. Same rules apply though: do your homework first on the model and then gauge a price for the car on the day and don't go over what YOU think the car is worth. Golden rule of buying ANY car - there is ALWAYS another one ... (unless it's a really RARE one).