I am sorry to learn about this situation.
As you have an account with this vendor, it is more likely than not that you also have a dispute resolution agreement with them (it should be one of the clauses in your account agreement, and/or in the sales agreement for the auction papers). Check this carefully as you will want to follow this procedure exactly if it exists.
If it does not exist, I would suggest attempting further negotiations in writing - telephone conversations are okay, but they are subject to differences in interpretation and simple misunderstanding. If you speak to someone by phone, promptly follow up with a "confirmation letter" to the other party summarizing what you discussed, any agreements you reached, and what further action is to be taken (if any).
You can file suit over "fraud in the inducement" - a false representation of material fact - which, if successful, would make the contract "voidable" (meaning you as the injured party can enforce or void the contract at your option).
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.