I'm Lucy, and I'd be happy to answer your questions today. I'm sorry to hear that this happened.
One of the few exceptions to a defamation of character claim is that a person has some protection when they are talking to someone where there's a common interest. If your former employer reasonably believes that you stole their equipment when you left, then they're allowed to tell your potential future employers for that company's protection. Meaning that, in order to bring a claim against them, you need to show that their belief is UNreasonable, that they know you didn't really sell the equipment, or that they are recklessly disregarding whether the statements are true or false (meaning they're not even trying to find out who really took those items). You could try sending them a "cease and desist" letter, explaining that you're only selling your own equipment and sold nothing, and that they must cease and desist telling lies about you immediately or you'll sue for the income they're causing you to lose.
Also, you said that they did a mass layoff with only two weeks notice. If your employer was laying off more than 500 people, OR 1/3 of the total workforce (if less than 500 people were let go) or if they're closing an entire plant, they're required by federal law to give 60 days notice or to pay you for the 60 days. That means, if you didn't get 6 weeks severance after being laid off, you may have a claim under the WARN Act. You could also send a demand letter asking for the money you're owed under that statute, or you could include it in the same letter mentioned above. That might help you get money faster than if you had to sue.
It's important that you are 100% satisfied with my courtesy and professionalism. Thank you.