Our computers and mobile devices increasingly carry our whole lives—work files, personal photos, tax information, and more. This makes used computers a powerful tool for opportunistic cyber-criminals; but there are ways to keep yourself and your data safe when it’s time to swap out your old computer. Here are a few rules to follow, whether you’re buying or selling, so you can move forward with confidence.

1. Never throw an old computer away

Not only are landfills a prime target for identity thieves, but most computers contain extremely toxic chemicals that can leach into groundwater. If your old computer is unusable and not worth the trouble of selling, drop it off at a local e-waste processing facility. They’ll responsibly dispose of the chemicals, recover valuable metals from the computer’s components, and ensure that the device is destroyed so that any residual information is completely unrecoverable.

2. Back up your data

Before scrubbing your old computer, make sure to back up all of your files—you might be surprised at what you find yourself needing later on, and hard disk space is getting cheaper every day. For this reason, you should try to keep your computer’s data organized, so you can just grab a few folders and transfer them to a flash drive in the event of sudden computer trouble. Just remember to wipe the flash drive once you’ve transferred the files to your new computer—having your backup on a portable device that you carry around regularly is a recipe for identity theft.

3. Carefully read any paperwork from a computer retailer

Just like buying a car, new is safer than used, so carefully read any contracts or agreements you’re asked to sign when buying a used computer. In many cases, small retailers may not accept any responsibility for malware that might be left over from the previous owner, so you won’t have any recourse if a registry key infection or other deep-rooted malware causes financial harm. To be safe, take the new device to a computer specialist you trust, and have them give it a thorough wipe. Make sure that the cost of these security measures doesn’t offset the savings from going used—sometimes you’re better off just looking for new laptops for sale online.

4. Don’t just reformat your hard drive

If you intend to sell your computer, reformatting your hard drive is a good way to make your personal information less accessible to identity thieves, but it isn’t foolproof—skilled specialists can often recover credit card numbers, account numbers, and passwords even from a reformatted hard drive. For the best possible results, AutoClave or DBAN (Darik’s Boot and Nuke) are the best freeware erasure options. If you want a software provider that guarantees their work, you’ll have to find a paid option, but DBAN and AutoClave are extremely secure.

5. Be wary of used computers with pre-installed software

You should be careful about any used computer with “free” software, particularly if you’re buying the computer from a private party online. A deal that seems too good to be true may be a Trojan horse for virus authors to access your personal information. A used computer that seems way beneath the price range for models with comparable specifications should raise red flags. Having said that, you can safely purchase almost any working computer as long as you completely clean it out before using it. For best results, install the erasure software from a flash drive or via a public wireless network to avoid passing malware to other devices connected to your home network.

Buying or selling a used computer can be a great option. When it’s time for an upgrade, practice good identity protection by keeping these security tips in mind.

Patricia Shuler is a BBGeeks.com staff writer from Oakland, California. She’s an admitted tech-junkie who’s quick to share her honest opinion on all things consumer electronic—including up-to-date news, user reviews, and “no holds barred” opinions on a variety of social media, tech, computer, and mobile accessories topics.