No, you cannot be forced to pay more than what you agreed upon. If EBay has a dispute with the value placed on the item by the seller, they can bring recourse against the seller, but you are entitled to the item as agreed upon in your purchase agreement.
If you do not receive the item as agreed upon, you can enforce your position.
Keep in mind, most purchases through EBay do require you to go through their internal dispute process prior to filing any sort of legal action in court, so carefully review your purchase agreement, if they do require alternative dispute resolution (claims filing, mediation, arbitration, etc.) follow that procedure exactly (such as giving notice, filing a written claim - whatever process is identified in your purchase agreement (you may need to follow a link to their dispute resolution site).
If there is no such claim (unlikely, but possible), then you are entitled to sue both the seller and EBay for breach of contract and request that the court order "specific performance" (require them to provide the item as agreed). Short of filing a lawsuit, you can try to mediate the dispute with them - contact your local bar association and request referrals to mediators, a third party neutral can often help you reach a mutually agreeable resolution. Use the bar association's referrals to contact a mediator or two, the mediator will then contact the other party to set up a mediation session, and you can go from there - hopefully resulting in a formal or written settlement agreement, and save yourself the time and expense of litigation.