There are few hard and fast rules. But, if I were looking at a used pack or tent, I'd check all the seams carefully, look for signs of wear, double check that the zippers run smoothly and aren't missing teeth (the sliders are often the first item to wear out on any gear), look carefully at the tent rain fly for signs the waterproofing is peeling off or worn out. You don't want gear failing on you mid-hike. If the gear is intact and fully functional, you're probably ok to buy it.

btw, I would never buy used foot wear, but I expect you'd know that much.