I have standard rules for my auctions:
Contact me within 48 hours after the auction closes (if you don’t just pay straight away), just to let me know you’re serious about buying the item and to arrange payment terms if necessary.
Pay within 7 days of auction close unless other arrangements have been made.
I have had a couple of buyers renege on their wins, usually coming up with some lame excuse (they’re always low feedback buyers too) but by and large all of my buyers have been good and prompt with their payments.
As long as all of your terms are clearly stated in your auction then you’re covered. Shipping should be done at the earliest convenience after the auction closes, unless you are a higher volume seller, in which case it is acceptable to have set shipping dates once or twice a week, particularly if you use UPS or someone like that who schedule pickup times.
I’ve never been hit with a PayPal scam, but I’m pretty diligent in checking out buyer histories to see how many negs they have, how new they are, what sort of things they’ve bidded on, etc. I don’t do this to all buyers, mostly just newbies, and if I get a weird vibe from any of them I treat their bids with caution, and will cancel any that set off red flags. I also have my account set not to accept bids from users with less than 0 feedbacks, who are not from a country that my auctions state that I deliver to, or who have more than 5 bid retractions in the last month. I don’t know how many questionable members this filters out, but I’m sure it’s stopped at least a few from getting through.