Six things I check when buying secondhand, and you should too

I go through a lot of clothes. Most of what I reject is rejected for the same handful of reasons, and knowing them makes you a much better secondhand shopper, whether you buy from me or anywhere else.

These checks work in a charity shop, at a car boot, and on a screen. In person you get to use your hands and your nose. Online you are relying on photos and on whoever wrote the description, which is why I try to write mine the way I would want to read them.

1. The fabric, before anything else

Natural fibres tend to age better than cheap synthetics. Cotton, linen, wool and silk can look good after years; a thin polyester jersey often does not survive many washes. Where a care label is photographed I list the composition word for word, so you can judge for yourself instead of guessing.

Two fibres worth knowing about. Linen creases the moment you sit down, and those creases are not damage, they are just linen. Viscose feels lovely and drapes well, but it goes baggy at the elbows and seat faster than anything else I handle, so on a viscose piece I look hard at the knees and the bum before I list it. Acrylic knits can look fine on the hanger and feel scratchy and flat in the hand, which is worth remembering when you are buying jumpers you cannot touch.

2. Seams and hems

Turn the garment over in the photos if there is a shot of the inside. Loose threads at a seam are fine and take two minutes to fix. Seams that have already started to open, or a hem that has been taken up unevenly, are a different matter.

On anything with stretch in it, the side seams are where the strain lands. Hold the fabric either side of the seam and pull gently. If you can see the thread stretching and daylight appearing between the stitches, that seam has already been worked hard. I also look for a seam that has been resewn in a different colour thread, because it usually means the original gave way once already.

3. Zips, buttons and fastenings

A broken zip is cheap to replace and should not put you off a good piece. A missing branded button on an otherwise perfect coat is harder, because matching it is often impossible. Missing belts are worth asking about, since they are frequently lost separately from the garment.

I run every zip up and down twice before it gets photographed. The failure I see most is a zip that closes fine but slides open again on its own, usually on trousers, which is why I fasten and test the whole waistband on anything in trousers and jeans. Hooks and eyes above a zip go soft and stop gripping. Poppers on lighter pieces work loose and pull straight through the fabric, leaving a small hole you can see if you hold it to the light.

4. Pilling and thinning

Knitwear pills where it rubs: underarms, sides, and where a bag strap sits. Light pilling comes off with a fabric shaver. Thinning, where you can see light through the fabric, does not come back, and that is what moves a piece from Good down to Fair in my grading.

Cuffs and the bottom hem band are the other places to look, because that is where a jumper gets tugged. If a cuff has lost its spring and hangs open, no amount of shaving fixes it. A cheap battery fabric shaver is one of the best things I own; I go over most of the knitwear before it is photographed, and I say so rather than pretending it came to me perfect.

5. Smell, which nobody mentions online

Smoke and damp are the two that do not wash out easily. I do not list anything that smells of either, and if I cannot get a smell out of something it goes to fabric recycling rather than onto the site.

Old perfume is the third one, and it clings to wool and to anything lined. A few days hanging outside in dry weather shifts most of it. If you buy in person, get the garment up to your face at the underarm and the neckline, which is where any smell lives.

6. Sizing, the one that catches everyone

The label number is a starting point and nothing more, especially on older pieces, which often run a size smaller than the same number today. This is why I offer measurements on any item before you buy. It takes me a couple of minutes and saves a return for both of us.

The quickest thing you can do is measure something you already own and wear happily. Lay it flat, measure pit to pit across the chest, then the waist, then the length from the shoulder seam down. Write those three numbers in your phone. Then you can compare against a measurement I send you instead of trying to remember whether you are a 12 in that brand. It matters most on fitted pieces, so I would always ask before buying occasion dresses or anything with a zip up the back.

The bits people forget to check

Linings fail before outer fabric does. Look at the lining under the arms and at the back vent of a jacket, because a shredded lining is a real repair job.

• Elastic waistbands. Stretch and release. If it does not spring back, it will not hold you either.
• Deodorant marks. Pale patches or stiffness at the underarm on dark fabric. These often will not shift.
• Hanger fade. Colour lifted along the tops of the shoulders from years in a sunny window.
• Collars and cuffs on white shirts. Grey at the fold is the giveaway on shirts and blouses.
• Alterations. A shortened hem or taken-in waist changes the fit away from the label, so ask for measurements rather than trusting the size.

What is worth repairing

Zips, hems, a loose button, light pilling: all worth it. Stains of unknown origin, thinned fabric, stretched-out elastic: usually not. When something needs work, I say so in the listing and price it accordingly rather than hoping you will not notice.

My rule of thumb is that a repair is worth doing if it costs less than the piece would cost you new and takes under an hour. That covers most fastenings and most hems. It does not cover replacing a lining or reknitting a cuff.

If you want to see how the grades work in practice, how we grade condition spells out all five, and everything currently in stock is in women's clothing. Anything you are not sure about, ask me before you buy.