If I decide to buy a used outboard, is a dyno run at full power the best way
of assuring myself the engine is OK?

If I wanted a seller to have a dyno run done on a candidate engine, I assume
I would be obligated to pay the associated costs. Is this normally the
case? I would like to set it up to where I wouldn't pay costs if the engine
failed the dyno run.

Is it something reasonable to ask of a seller, assuming I say I'll pay the
cost unless it fails?