As far as i remember tups are insured for 30 days after sale against death and there is usually a garauntee that he will 'work'. However 'work' may consist of getting just 1 ewe in-lamb, which isn't ideal if he gets put with 40 ewes..if he fails to work the seller either repays the purchase price or provides a similar value replacement
having said that 've only heard this in theory, Thankfully i've never had to make a claim, but i do have a tup that only manages 5 or 6 ewes every year so he just gets used as a teaser two weeks before the main tups go in