I used to think people were crazy for grabbing mattresses off the curb...
Then I found a near-mint Sealy with a waterproof cover under a tarp in Denver last fall, and after a deep clean with my Bissell it's been comfier than the one I paid $800 for.
Did you check under the tarp for any spiders or critters before hauling it home? I feel like I would have shrieked like a cartoon character and then still taken it because a good mattress is a good mattress. My brother once grabbed a curb mattress that turned out to be absolutely pristine except for one small mystery stain that we decided was probably just coffee. I teased him for years but now I'm the one bragging about his find to anyone who will listen. Guess I'm the crazy one now.
Did you at least give that tarp a good shake before you grabbed it? That's my #1 curb mattress rule. I do the "smell test" first too if it's been rained on at all. And @sarahsullivan you're not crazy, you're a smart shopper. That mystery stain was probably coffee, I've found more than a few "coffee stains" on curb scores that turned out to be nothing. Pro tip: bring a flashlight and a garbage bag to wrap it tight for the ride home just in case you find a surprise later.
My sister found a leather couch on the curb last summer that looked like it had barely been sat on. She was so proud of it until she flipped it over and found a half-eaten bag of chips and a toddler shoe stuck in the springs. We still joke about the "couch chips" but honestly after a good vacuum and some Febreze that thing was nicer than anything in her living room. Makes you wonder what people are throwing away sometimes.