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You question is a little vague, but the general rule is to make sure that your add is truthful. So long as that is the case, you can pretty much say what you want.
I hope this helps.
Unless you are a merchant, private sales are, by law, "as is". So long as you make no promises you have created no facts upon which a buyer could rely. For instance, you can safely say "really nice RV" but don't say "guaranteed to make you happy" or "never a problem with it.
In most cases, the less you write, the better.
If you are a merchant, then at minimum there is an implied warranty of merchantability that you cannot disclaim. So long as your ad is otherwise truthful, you are on about as good a footing as you can get.