Almost every other mistake in this category can be fixed in the next listing. This one cannot be fixed at all.
Cleaning a coin
A polish, a wire brush, a dip, even an enthusiastic rub with a cloth. It removes surface the coin has had since it was struck, it is immediately visible to anybody who knows the series, and it is permanent.
If you have already done it, say so. A cleaned coin honestly described is a cleaned coin sold.
Assuming age means value
Common old coins survive in enormous numbers. Rarity sits within a series — the mint mark, the year, the variety — not in the century. What coins are worth.
Not identifying the exact issue
Year and mint mark are what a comparable has to match. "Old silver coin" is not a listing anybody can bid on with confidence.
Grading confidently without grading
The trade has a numeric ladder and people who use it daily. Give a band and say you are not a grader — it is believable and it protects you.
Posting loose in an envelope
Coins in a padded envelope arrive scuffed against each other and sometimes not at all. Each one in its own holder, rigid, boxed. Packing coins.
For notes: ignoring the folds
Paper money is graded largely on folds and crispness. Photograph flat, in even light, and count the folds rather than describing them vaguely.
The one that costs most
Still cleaning. It is worth repeating because every week somebody does it the night before a show, trying to help.
Questions people ask
What if a coin is already dirty?
Leave it. Dirt is not damage; cleaning is.
Should I sell a collection as one lot?
Identified pieces individually, the rest as lots.
Does a holder add value?
A graded holder carries the grade. A plain one is storage.
More: selling coins, describing condition.