/// Why a step towards a sale was turned down.
public enum TradeError: Error, Equatable, Sendable {
    /// The amount is not a positive number of cents at or under the asking price.
    case invalidAmount
    /// The listing does not exist, or is sold.
    case listingUnavailable
    /// The account is not a party to the conversation, offer, or sale — or the step is not theirs to take now.
    case notAllowed
    /// The buyer already has an open offer on the listing.
    case offerAlreadyOpen
    /// The offer is under the floor the seller hears offers from, in euro cents.
    case offerTooLow(floorCents: Int)
    /// The listing's seller takes no offers.
    case offersNotAccepted
    /// The listing, shop, conversation, or sale belongs to the account itself.
    case ownListing
    /// The shop has no owner to sell on its behalf.
    case shopUnattended
}