Cinema paper is unforgiving, and most of the damage is done by a seller trying to help.
Flattening a folded poster
It is restoration, it can crack the ink, and it is the buyer's decision rather than yours. Post it folded, flat between boards.
Never showing the reverse
Tape, backing, repairs and old mounting all live there and none of it faces the camera. Buying cinema memorabilia is what the other side is reading.
Naming a format instead of measuring
Poster formats vary by era and studio. Measure the sheet in centimetres and put the number in the listing.
Selling a reproduction without the word
Reproductions sell perfectly well described as reproductions. Sold as originals they are a counterfeit problem, not a grey area. Counterfeits policy.
Not declaring linen backing or repair
Both are normal and accepted. Undeclared, they end the sale when the buyer unrolls it.
Laminating anything
Ever, for any reason.
Rolling tightly into a narrow tube
A tight roll cracks ink and a narrow tube squeezes it. Wide core, loose roll, rigid tube, taped caps. Packing posters.
The one that costs most
Tape. On the piece, on the folds, "just to hold the roll". Old tape stains through and the stain is permanent even after the tape comes off.
Questions people ask
Are fold lines a defect?
Not if it was issued folded. Separation at the folds is.
Is a trimmed poster worth selling?
Yes, as trimmed, at trimmed prices. Measure it.
What about autographed posters?
That is a provenance claim and belongs in its own sentence. Provenance.