What?? 30 days?? That is an absolutely ridiculous thing to guarentee. There is a host of potentially bad things that could happen to an unbroke horse with a new rider in 30 days. Agreed, if they want a sound horse they need to get a prepurchase and see what's going on inside. The most generous thing I might consider doing is a one week "lease" with a hefty lease fee that could be deducted from the total purchase price at the end of the week if they decide to keep her. And I would only consider it if I knew the trainer/owner and trusted them whole heartedly. That is NOT a typical thing to ask of a seller. Since you have other people interested I'd tell them no.