How unfortunate! Just curious... why did you sell this turtle 2 days after buying it yourself? And, why would you guarantee it for longer than you've even owned it??
Questions aside, you have nothing to worry about. The poor tortoise will be the one to suffer if anything goes wrong... Glad to hear you've discussed husbandry with the purchaser, but shouldn't that have been done before the sale was final? (From the thread it appears this was not discussed before hand, at depth, and that the buyers are apparently oblivious to the basic needs of this tortoise...)
Anyways, hope it will begin feeding again...