A lot will depend on what was agreed between them at the time. A legally binding contract could be forced if there was an offer, an acceptance and consideration. For example, if the buyer had offered to buy the item for a set price, the seller accepted it and then the buyer showed consideration, such as travelling down on the promise of the sale, that could indeed have created a binding contract.
If the seller then breached the contract by not selling the item, they would be acting in breach of contract and the buyer could consider making a claim for compensation. However, any compensation must only be equal to any losses they have suffered as a result of this. They could consider claiming for the wasted travel costs, but apart from that they would only really be able to claim for a price difference between what they could have bought the item for and what they may have had to pay for an item with equal specifications.
The travel costs will only likely cover actual costs like fuel, not time. There is general compensation they can claim, unless they can show losses so that 250 may not be payable. The other 250 could potentially cover any extra expenses but only if this was a similar spec trailer, they cannot go and buy something much newer or better and then claim that difference. So whilst there MAY be a claim, it is unlikely to be as large as what they are after.
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