Buying a used phone is a great idea. It's economical, it's good for the environment, and if you don't need or want the latest Big New Thing (and they're all big these days), used or refurbished is the way to go.

But there can be a lot of uncertainty when purchasing a used device—particularly around whether it's being sold legitimately. Last year alone thieves stole a record 3.1 million smartphones, and you can bet they wouldn't keep stealing them if they weren't pretty easy to sell later. With that in mind, here's how to make sure the used handset you buy is on the up and up.

Select a seller carefully
First, you want to buy from a reputable venue or seller. Carriers offer used and refurbished phones on their websites. For better deals though, Amazon, eBay, and Craigslist are typical go-to online venues. But while they may have some great prices, they can be lacking in the reputation department. Amazon and eBay at least have a safeguard of profiles and seller reviews that make them more trustworthy than Craigslist. Swappa and Glyde, other online destinations for buying used phones, also have policies and safeguards in place for buyers.

Check the numbers
Listings, no matter the site, should feature some basic stats about the device being sold. Color, correct model number, and storage capacity are must-haves in the device description, as is a description of the overall quality (things like "good as new" or "a few scratches on the back"). It's best if the listing shows a photo of the actual smartphone, rather than an official product shot of that phone model. If you can contact the owner directly, ask for additional photos of the phone to make sure it matches the product description. You'll also want to ask for the IMEI (International Mobile Station Equipment Identity) or serial number of the device. If the seller is unwilling to share this info, that's a big red flag.

For potential iDevice owners, Apple has a new service on called Check Activation Lock Status that lets you quickly see if a device was procured illegally. By entering the IMEI or serial number of the device you plan to purchase, you can tell if someone used Find My iPhone to switch on Activation Lock. Activation Lock, introduced in iOS 7, locks down the device, making it unusable until the correct Apple ID and password have been entered. Thus, if you make the mistake of purchasing an iOS device that was stolen, you won't be able to use it. You can also check how much warranty is left on the device using the serial number, too.

Carriers and Swappa should also let you check the IMEI of a device before purchase (especially if you're willing to make a phone call). Glyde, which doesn't currently give you the option to check ESN beforehand, holds the seller's payment in escrow until the buyer has checked the device and confirmed that it's clean and has a full refund policy. You can enter the serial number in here or here to check if a phone, regardless of make or model, has been reported stolen.

Know your return policies
Once you've determined your phone of choice is legit, it's time to pony up. This is straightforward on most platforms with the exception of Craigslist (but be sure to read up on return policies first, just in case). If you've decided to buy a phone off Craigslist, it's a good idea to meet in a public location like a carrier store so you can activate it following your purchase, with the seller there. If they already gave you an IMEI or ESN, you'll want to double check that it matches up before handing over any cash. And if they didn't give you that information before, you can check the serial number with one of the sites we mentioned above once you have it in hand.

Now all you have to do is decide what to do with your old phone.