Clothing sells on fit, and a label size is not fit.
Giving the size and nothing else
Sizes are not comparable across makers, countries or decades. Measure the garment flat — chest, shoulder, sleeve, length — and put the numbers in the listing. Selling menswear.
Not declaring alterations
A garment taken in or hemmed does not match its own label. It is not a fault; it is a fact that changes what the measurements mean.
Skipping the composition label
Fabric decides drape, warmth, shrinkage and washing. Quote the label, or say it is missing.
Photographing on a hanger in a dark room
A garment flat, in daylight, with the tape on it. That is the listing.
Not checking the usual wear points
Collar, cuffs, underarms, elbows, hems. A buyer checks them first, so you should.
Not mentioning smell
Storage, smoke and damp survive a parcel and are a return every time.
Packing without a sealed bag
Clothing survives almost anything except damp, and the monsoon finds every parcel. Packing menswear.
The one that costs most
Mixed-size lots. Parents and buyers shop by size; a bundle of assorted sizes is a gamble, and it sells like one. One size per lot.
Questions people ask
Should I wash before selling?
Yes, by the care label, and say that you did.
Does a small mark need mentioning?
Always. Small marks look hidden on purpose when found.
Is branded clothing worth more second-hand?
In a narrow part of the category, and only with condition and provenance.
More: describing condition, what it is worth.