Half the mistakes here are about the claim and half are about the parcel.
Sounding more certain than you are
A seller who says "I cannot confirm it" is trusted more than one who cannot imagine doubt. Say which rung of the provenance ladder you are on. The ladder.
Not showing the certificate's issuer
A certificate is worth exactly what its issuer is worth. Show the name, not just the paper.
Mixing replicas and match-worn in one description
Say which of the three it is — worn, issued or replica — in the title.
Not checking length pricing before quoting
Bats are long, most couriers price on length as well as weight, and some refuse them. Finding out after the sale means the difference comes off your own balance. How delivery is charged.
Padding directly against a signature
Pressure and heat lift ink. Acid-free tissue over the signature, and nothing pressing on it.
Posting a framed item
Glass that breaks damages the thing it was protecting. Offer collection or sell it unframed.
Folding a shirt on a new crease
Fold along the folds that already exist, and say how it was folded.
The one that costs most
Washing a match-worn shirt before selling. It cannot be undone, it changes the object, and it must be declared if it has happened.
Questions people ask
Is a signed stump or ball harder to post?
Awkward rather than fragile. Pad the extremities and check the size with the courier.
Do programmes and tickets sell?
Yes, and they are easier — paper condition is most of it.
Should I get something authenticated first?
It depends on value and on who would do it. Say plainly what you did and did not do.
More: what it is worth, packing it.