The days of groundbreaking advances in smartphone performance and features are largely over. Today's smartphones may be the best ever, but they're not that much better than the models from a year or two ago.

That makes this a good time to consider shopping for a used smartphone. Refurbished iPhones and Samsung Galaxy phones sell for around $300 less than the newest, not-much-better versions of those models on the manufacturers’ sites. And you can find even better deals by hunting around on BestBuy.com and other electronics retail outlets, or even on eBay.

But there are some pitfalls to avoid when buying a used smartphone. The following tips can help. (Check our advice on how to sell a used smartphone.)

Check Network Compatibility

You can’t use all used smartphones with all of the cellular carriers.

The big dividing line is between the CDMA networks used by Sprint and Verizon and the GSM networks used by AT&T and T-Mobile. As a rule, if you're buying a used phone, plan to stick with the same type of network the phone started on.

Figuring this out will be easy if you’re buying a phone from the carrier or manufacturer but may be trickier if you’re shopping with another seller. Be sure to check the product specs carefully.

One surefire strategy is to buy a phone that has the hardware needed to work with all U.S. cell providers. On the Android side of things, these phones include the Google Pixel and Pixel XL. For Apple fans, the iPhone 7 and 7 Plus models originally used on the Verizon and Sprint networks can migrate to almost any carrier. (The same iPhones sold by AT&T and T-Mobile are marooned on GSM networks.)

A nifty web tool called willmyphonework lets you to check the compatibility of quite a few phones. You’ll need to type in the phone’s original cell provider, model number, and the provider you plan to use. The site will tell you whether the phone will work and also whether it will be able to access your carrier’s fastest network (4G). That’s not a given.

Make Sure It Wasn't Reported Stolen

Today’s smartphones have anti-theft features that can cause problems for someone buying a used smartphone, especially if the seller is a person rather than an established company.

On the iPhone, the feature is called Activation Lock. It lets a user “brick” the phone if it’s been lost or stolen. Android smartphone users have a theft-deterrent as well. It's called Android Device Manager and allows users to locate, lock, or erase the data on a lost or missing phone.

Make sure these tools are deactivated on any phone you’re thinking of purchasing. You can do that by going to a site called imeipro. All you’ll need is the IMEI number—the acronym stands for International Mobile Equipment Identity, and every smartphone has a unique one. If you're checking out the phone in person, you can find the number somewhere in Settings or (with some phones) by dialing *#06# on the keypad. Do that and the IMEI will pop up on the screen.

You also have to make sure a used smartphone isn't locked to the old user's cellular carrier and account. This has nothing to do with network compatibility or thefts. Providers lock phones to their own networks to make sure phones get paid off in full by consumers—and perhaps to add some friction to the process of switching to another company.

The online tools we've mentioned won't tell you whether a phone is still locked to its original carrier. You just need to bring it to your new carrier and see whether the salesperson can set it up. And this is one reason it's wise to buy your used phone from a reputable seller, one with a good return policy.

Check for Water Damage

Excessive moisture can damage or kill a smartphone, but the damage might not show up right away. There’s a way around this: You can check the tiny moisture indicator that’s built into almost every smartphone to help repair centers diagnose problems.

On phones with removable batteries, the indicator is typically a tiny white patch behind the battery or near the SIM-card slot, sometimes with a pattern on it. If it ever becomes wet, the indicator changes color, usually to a solid red. And it stays that way even after drying out.

With phones that don’t have removable batteries, including iPhones and newer Samsung models, the indicator is often in the SIM tray opening. To find out for sure, do a Google search with the name of your phone and the phrase “moisture indicator." Note: You may need a flashlight and a magnifying glass.

Obviously, this is something that’s best done in person before you buy a used smartphone. But if you're ordering over the web, check this as soon as the phone arrives in the mail.

Once again, it's smart to buy from a vendor with a good return policy.