One rule underneath all of this: nothing should be able to move inside the box, and nothing should touch the wall of it.
Most breakages are not impacts. They are an object sliding into a corner for six hours.
By material
Ceramics and glass. Wrap, then suspend. The item must not touch any wall — two to three centimetres of give on every side. Box inside a box for anything you would be upset to lose.
Framed work. Tape a cross over the glass so that if it breaks it holds together instead of cutting the thing it is protecting. Corner protectors. Never flat under weight.
Textiles. Fold along existing folds, never a new crease, and bag against damp. A pressed-in fold line can be permanent.
Paper — comics, posters, prints. Rigid backing, or a tube. Never folded, never loose in a soft envelope.
Cards. Sleeve, toploader, then taped so the toploader cannot slide. Then a rigid envelope.
Electronics. Battery rules apply and couriers enforce them. Anti-static for anything exposed.
Food. Sealed, cushioned, and honest about how long it survives.
Finishing
Tape every seam. Put the address inside the box as well as on it. Photograph the packed parcel — it costs ten seconds and settles arguments.
Questions people ask
What should I buy to start?
Bubble wrap, a roll of good tape, assorted boxes, corner protectors if you sell framed work.
Can I reuse boxes?
Yes, if they are rigid and you cover old labels.
Who pays if the courier damages it?
The claim is yours to make. The buyer is covered by us meanwhile.
Is it worth photographing every parcel?
Yes, for anything valuable.
Next: what comes out of a sale.