People have lives outside of paintball, like jobs, family and responsibilities. I usually give the two week rule. Or if I feel generous and I'm told before hand that it will be shipped late, the 45 day limit before I can't file my paypal claim anymore.
One general way I can tell how fast or slow something comes is the amount of feedback. If the guy barely sells things from his past posts, it's probably not a routine thing for him to go out to the post office on a frequent basis. So those usually end up slower as expected.
When you buy used, you take that gamble of good faith that the other person delivers. There's no obligations. Want full service? Pay store prices (and even that's not guaranteed).