When you first go to look at a bike, let the seller know not to crank it and warm it up. A warm engine is easy to start and can hide many tuning issues. A cold engine, however, can show the weak links in the chain.
Have a reason why you want to pay less money. For example, list out the parts or the back fees, and express to them your budget.
Compliment the item and if you make them a lower offer than the asking price, don’t insult them. Make it clear that it’s how much you can spend, not how much you personally think it’s worth.
Be friendly and don’t waste the person’s time by rescheduling and showing up late.