I've bought and sold so many used boots over the years it has become my primary means of acquiring boots. The upside of buying used boots is being able to punch way above your budget, getting a better boot for the dollar while minimizing potential loss should the boots be a catch and release for your due to fit issues, etc. If you buy new boots from a retailer with a decent return policy you typically can try the boots on inside your home and should they not fit can return them and possible eat the return shipping cost and hopefully not a restocking fee. With a used boot you can try actually out the boot in the intended conditions and if it doesn't meet your requirements (and you didn't overpay) you should be able to sell them for minimal to no loss.
The downside of buying used boots, naturally, is buying someone else's problem but you can mitigate that risk by several means. 1. Stick with boots that have minimal to no visible wear, typically catch & release boots due to fit issues. 2. Don't buy old boots even in new condition. With age comes dry rot and delamination. You don't want to be side-hilling with a sole flopping around miles from the nearest duct tape. 3. Do your research on pricing and user reviews. 4. Buy from a reputable party and pay with PayPal goods & services.