I get what you mean about floating with thinset—it’s messy and eats up way more than you plan for. Personally, I’ve found that using self-leveling underlayment before tiling can save a lot of headaches, especially with mosaics or smaller tiles. It’s a bit of prep, but it means you’re not constantly fighting those little dips and rises. I’m not a fan of stacking extra backer board either; it always feels like you’re just creating more seams to worry about down the line.
On the Craigslist front, I’ve had mixed luck. Scored a brand new vanity once for half price, but then wasted hours chasing after a “like new” tub that turned out to be trashed. Sometimes the time and hassle just isn’t worth the savings, especially if you’re on a tight schedule. Still, for big-ticket items, I can’t help but check just in case.