Your clients will likely need to consult a local real estate attorney and to obtain a title report to both their parcel and the parcel they use to access their parcel in order to determine if there is an existing right of way. If there is not, the clients might have a claim for a prescriptive easement - which the sign your describe is meant to defeat.
I am not a CA attorney, laws vary from state to state, therefore you should always consult a local attorney.
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If seller's position is that he/she already has an easement across parcel A, then of course the no-trespass sign doesn't apply to seller, right? It only applies to others who don't have the easement. But the parcel A owner certainly has the right to have a sign even if there's an easement, so long as the sign isn't blocking the path. In any case, it sounds like an ugly mess that will end up in litigation. The seller should hire a property lawyer to send letter to the parcel A owner demanding access and perhaps explaining that parcel A owner could be sued for interfering (with the lock) with the easement that's existed for years and interfering with a prospective business advantage (yours and seller's) and maybe is clouding title, etc.
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I agree the only solution for your sellers may be litigation. If there is no recorded easement, there are other theories which may grant an easement to sellers but they depend on unstated facts. Although not the only theory that may be implicated, adverse use certainly is.
An easement acquired by adverse use may be extinguished by non-use for the requisite 5-year period. It appears that is exactly what the owner of Parcel A is attempting to do.
Seller's option is to retain counsel to establish easement rights if they do not appear in a grant and to obtain a temporary and permanent injunction to prevent the owner of Parcel A from interfering with those rights.
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