Trials will depend on the person or business selling it. It's super common to pay the person the full price of the saddle before taking it on trial (i.e., I've never seen anyone not ask for the full price up front), just in case someone decides to take off with it. Obviously if it doesn't work for you, you get a full refund of your money, it's just an insurance measure. The length of the trial will depend on your needs and the person/business. If you know that you may need a week or two to make sure it's a good fit, the seller can likely make it happen.
It's up to the discretion of the person selling it, so just asking the person you're interested in buying a saddle from is going to be your best bet!
Most people will let you pay for the saddle and try it for whatever amount of time you and the seller agree on. If the saddle doesn't work, you return the saddle and get your money back.
Or, find someone local and they will usually come out and let you try it when they're watching. If you like it, you pay for it. If you don't, then they take their saddle home.