Vintage sizing has almost no relationship to what you wear now. A 1960s size 12 is roughly a modern size 6, and sizing was renumbered repeatedly through the twentieth century, so the number on the label tells you when the garment was made rather than whether it will fit.

The only numbers that matter are the flat measurements: bust, waist, hip, shoulder to shoulder, and length. Good sellers list all of them, taken flat and doubled. Take the same measurements of a garment you already own and love, and compare those, not your body. Two centimetres of ease at the waist is the difference between wearable and not.

Check the shoulder seam first, because it is the one thing that cannot be altered cheaply. A waist can be let out or taken in, a hem can move, a shoulder cannot. On a jacket, if the shoulder is wrong, the piece is wrong.

Then ask about the fabric's give. Bias-cut silk and knits accommodate a little; woven cotton and structured evening pieces accommodate nothing.

Assume everything is described optimistically and ask questions in writing. Excellent vintage condition means excellent for its age, not new. What you want to know specifically: are there underarm stains, is the lining shattered, are there holes at the seams, has it been altered, does the zip run, and is there any odour.

Old silk is the main risk. Silk from before the 1970s can be shattered, meaning the fibres have degraded into fine splits that cannot be repaired and will spread. Ask for photographs of the underarms, the seat and the hem in daylight. Sequins and beading are the second risk, because the thread perishes and a heavily beaded 1920s or 1980s piece can shed permanently.

Alterations are not a problem in themselves, but they change the fit in ways measurements can hide. Ask what was altered and whether the original seam allowance is still there.

Finally, ask what it smells like. Smoke and mildew are the two things that most often cannot be fixed.

Pay for provenance you can verify and for pieces from a designer's defining period. A dated runway piece with a documented season is a different object from an unlabelled dress of the same decade, and it holds its value in a way that anonymous vintage does not.

Pay for condition on anything delicate. On evening wear the premium for a piece that has genuinely been looked after is small compared with the cost of restoration, and restoration on beading or degraded silk is often impossible.

Do not pay a premium for a label alone. Plenty of vintage carries a famous name on a garment that was mass produced at the time, and the interesting buys are frequently the unlabelled ones with better cutting.

On practical costs: budget for tailoring on almost everything, factor in import duty on anything crossing a border, and check the return policy before you buy. Many vintage sellers are final sale, which is exactly why the measurements conversation has to happen first.

Buying vintage online works, but only if you treat it as buying one specific object rather than a size. The people who end up disappointed are the ones who bought a decade and a designer and hoped.

We would spend the money on daywear from the 1960s and 1970s, where the cutting is excellent and the prices are still reasonable, and be much more careful with fragile evening pieces unless the condition report is detailed and the photographs are honest. We would also budget roughly a quarter of the purchase price for alterations, because a vintage piece that has been fitted to you looks far better than a new one that has not.

If you take one thing away: get the flat measurements and compare them against a garment you already own, because that single step prevents most vintage mistakes.