if in fact the seller signed the acceptance offer and faxed it only to the listing agent and it has not yet been transmitted nor received by the selling agent or their buyer, than the seller does have the right to pull back the reigns of the initial acceptance and consider a higher offer. the seller could also respond with a multiple counter offer (listing agent must notify all selling agents if this were to happen) with the hopes of gaining a higher sales price or just accept the highest offer without a counter. once the selling agent and/or buyer has received the acceptance from the seller and is initialed by the selling agent and returned to the seller, then and only then is it a done deal.
Once the offer is accepted the seller can not accept another offer. If the agent has that represents the seller has confirmed they have a signed acceptance that should be enough to ease your concerns. It is the agent's duty to deliver the signed acceptance from the seller to the buyer (if they are also representing them) or the buyer's agent.
The Reavis Group
Keller Williams Wilshire