I think this is incorrect for a number of reasons. First, your reference to "all real estate contracts must be in writing" is a reference to the statute of frauds, which does apply to contracts for the sale of land. However, I doubt that the sale of Wyndham points is a sale of land that would fall within the statute of frauds, although as a sale of a timeshare contract, it would likely be subject to a right of rescission under state law. Even if the statute of frauds were to be applicable, I think the specificity of the eBay ad would satisfy the writing requirement, and that OP's offer to purchase and clicking the bid button would suffice as a signing. Moreover, it looks like OP did MORE than merely click a bid button, as it appears he made a specific best offer for a lower purchase amount ($8,500), which the seller accepted. So I do think there would be an enforceable contract. Of course, the shortest answer to all this is if the buyer were to back out, the greatest likelihood is that he would simply get negative feedback because I don't think any sellers are likely to sue buyers to enforce such a contract.