In answer to Arsenic - it seems you are not quite clear as to the matter causing the disagreement.
And, although I have my own opinions, I am not posting for that reason - rather to get a consensus from others of what, if anything, can be done to resolve matters without too much angst.
Horses become unfit for purpose all the time, for many reasons, no-one is disputing that fact. The issue here, I understand, is one of misrepresentation - and good faith.
Upon arrival, the area of concern was brought to the attention of the vendor's agent who confirmed a vet check had been performed and the horse passed fit.
Why, under those circumstances, would the purchasers spend more money to repeat the process when they trusted the agent?
They took said agent's word as being in good faith and this is where the problem has arisen, it seems.
On-line auctions are a cheap option to move horses on, and the system can only work well if full disclosure is made when advertising.