Secondhand buying has moved from necessity to mainstream, and the skills involved are genuinely different from buying new, because the variable you cannot control is condition.
What cannot be fixed
Establishing this first saves a great deal of disappointment.
Fabric that has thinned across an area — not a hole, but visibly worn — will fail there and cannot be restored.
Permanent stains, particularly oil-based ones that have been through a dryer, are generally set.
Shoulders that are too wide on tailoring.
Elastic that has perished, which affects waistbands and stretch garments and cannot be replaced without rebuilding.
Odour that has penetrated the fibres rather than sitting on the surface, particularly smoke in wool, which sometimes cannot be removed at all.
What can be fixed cheaply
Missing buttons, obviously.
Failed zips, by replacement.
Hems, seams and small holes.
Pilling, with a fabric shaver.
Most surface stains, if identified and treated appropriately rather than scrubbed.
Fit, within the limits of the seam allowances.
Which means an item with cosmetic problems and sound fabric is a considerably better buy than one that looks clean and is worn thin.
The inspection sequence
I do the same check every time, in the same order, and it takes about a minute.
Hold the garment up to light and look through it at the elbows, seat, knees and underarms. Thin areas show immediately.
Check the underarms for staining and stiffness.
Run the zip fully, both ways.
Look at the inside seams for splitting or previous repairs.
Check the collar and cuffs for wear at the folds.
And smell it, which is unglamorous and catches the problem that is hardest to fix.
Buying online
Where the difficulty concentrates, since none of the above is possible.
Measurements are the substitute, and asking for flat measurements of a specific garment is reasonable and most sellers will provide them.
Photographs of the areas that fail — underarms, elbows, seat — are worth requesting, and a seller who declines has told you something.
Return terms vary enormously between platforms, and on some the buyer has essentially no recourse, which should be reflected in what you are willing to pay.
Vintage sizing
Older garments follow entirely different sizing conventions, and the drift in labelling means a vintage size is generally considerably smaller than the same modern number.
Which makes measurements the only useful information on anything older than a couple of decades.
Construction is frequently better on older garments, since the labour cost calculus was different, and fabric quality varies more than people assume.
What is worth paying for
Coats and tailoring, because construction quality matters most there and because the labour to make them well has become disproportionately expensive.
Leather goods, where the material improves with age.
Knitwear in good condition, which is expensive new and readily assessed for pilling and shape.
Denim, which was designed to be worn in.
What is not
Anything with elastane, which will have degraded regardless of appearance.
Anything close-fitting, where alteration scope is limited and fit is critical.
Footwear, mostly, since it has moulded to a different foot and the interior wear is not assessable.
The honest framing
Buying secondhand is cheaper and lower impact and it takes considerably more time.
The people who do it well treat it as an ongoing search rather than as a shopping trip, which is why they find things and occasional browsers do not.
Washing an unknown garment
The first thing to do and the easiest to get wrong.
Care labels may be missing, illegible, or wrong if the garment has been altered.
Which means identifying the fibre first, by feel and by burn test if necessary, then treating it as the most delicate plausible option.
Cold water and gentle handling will not damage anything. Hot water and agitation will destroy several things irreversibly.
Testing colourfastness on an inside seam with a damp white cloth takes a minute and prevents dye transfer disasters.
Odour removal
The most common obstacle and it is frequently solvable.
Airing outdoors, out of direct sun, removes a surprising amount over several days and costs nothing.
Enzymatic treatments address biological odours specifically, which ordinary detergent does not.
Smoke in wool is the hard case, and professional ozone treatment sometimes works where nothing else does.
Where to actually look
Charity shops in wealthier areas have better stock and higher prices, which roughly cancel out.
Dedicated vintage dealers charge for the curation, which is a real service if your time is worth anything.
Online marketplaces have the widest selection and the least assessable condition, which is the trade.
Estate and house clearance sales are where the genuinely underpriced things appear, and they require patience and turning up.
Pricing sanity
Worth a note since the market has changed considerably.
Secondhand prices for desirable items now frequently exceed what comparable new items cost from lesser-known makers.
Which means the automatic assumption that used is cheaper no longer holds, and comparing against new alternatives is worth doing.