I love Levis 516 jeans. The fit and denim are unlike anything you'll get elsewhere. When you find something close in another brand or style, seriously consider them.
If you're buying used or 'pre-owned', first and most important use your eyes. The best are an arctic blue with pronounced fade lines around the crotch and knee area. Tight fit at the top with an obvious flare from the knee down.
Be dubious of 'new'. It's possible but worth thinking twice about.
Expect wear, damage - they're second-hand. It doesn't mean they're sub-standard, it enhances the look of this product.