If you’ve been around the digital block a few times, you know to be careful when buying online. Some sellers, a distinct minority according to the evidence, are unscrupulous.

But you also need to be careful when selling or trading in. And it’s not only the small percentage of unscrupulous buyers you need to watch out for. Sometimes it’s the particular online marketplace itself.

In the old days, when you had stuff you no longer needed that you wanted to try to get some money for, you could do a yard or garage sale or take your items to a second-hand or consignment shop. You can still do this today.

But there are now digital options that can be more convenient and lucrative. Among the options are Craigslist and Facebook Marketplace. Both can hook you up with local buyers, preventing you from having to ship. Another option is eBay.

Still another option is Amazon Trade-In, which specializes in letting people trade in used smartphones, tablet computers, notebook computers, and other electronics.

You might think Amazon would be a great choice here, with its stellar overall reputation for reliability. Amazon has been around for more than 20 years, and it’s by far the largest online marketplace in the US.

Amazon has always done of a good job of providing feedback to shoppers about the reliability of sellers, which can be businesses or individuals. About 40% of the merchandise sold is from third-party sellers, with the remainder from Amazon itself.

Amazon’s overall reliability, however, hasn’t prevented unscrupulous people from writing bogus positive reviews about their own items or bogus sabotage reviews about others’ items, though Amazon devotes considerable resources to cracking down on this.

Some people disapprove of what has been reported to be tough working conditions for Amazon employees. Workers have been tracked on a minute-by-minute basis and threatened with reprimands if they complain. This fits in, however, with Amazon’s image of efficiency, even if it sometimes is harsh.

I personally use Amazon all the time to buy products and consistently have had positive experiences, from the quality of the items to the fast shipping times. I also like Amazon Smile, which lets you designate a nonprofit organization that Amazon will donate a percentage of revenue to.

Amazon Trade-In, however, has its problems. Announced in 2009, it initially received positive reviews. When you send in an item, Amazon has you rate its condition and check a box to choose whether to reject the trade if Amazon thinks it’s in worse condition or accept the trade at a lower condition, if Amazon determines this, and a subsequently lower amount of credit. In return for the item, you receive an Amazon gift card, good for buying anything on the huge marketplace that’s Amazon.

After a few years, however, stories began to surface about Amazon unfairly downgrading the condition of items people trade in. ZDNet reported this in October 2015 in an article titled “Amazon Trade-In: Fair Value for Your iPhone or Scam?” According to the article, “It is more or less standard practice for [Amazon] to automatically downgrade the condition of the device no matter what condition it is in.”

Lifewire reported the same in an article and several updates of it, the latest in October 2016, with the article titled “The Precipitous Fall of Amazon’s Trade-In Service.” According to the article, if you complain too much about this practice with your own trade-ins, “Amazon can and will just outright ban you. Not just from trading, but from using the site at all.”

A new investigation by freelance tech journalist Andrea Smith (andreasmith.org) just found that the same thing is still happening.

After repeatedly having items downgraded by Amazon, out of frustration Smith did a test. She sent Amazon Trade-In an unopened Roku 4 streaming device, sealed with the plastic wrap still around it, as she told me in email. She rated it as being in excellent condition, and she chose “reject the trade” if Amazon didn’t agree with the condition. Amazon didn’t agree and sent it back.

She phoned to ask what the issue was and was told there were scratches on the device. She looked for the scratches but couldn’t find any. Amazon wanted to offer her 45% of what it would have if it determined that the item was in excellent condition as she said.

Smith says, “Trader beware!” She recommends going with another trade-in site. Other sites include Gazelle, Swappa, SecondSpin, and GameStop.