Jawbone fitness trackers are no longer for sale at Amazon, Selfridges and GroupOn after a Which? investigation.
The consumer group found that the trackers were no longer supported, so anyone buying the gadget would be unable to measure their progress.
The consumer group investigated after seeing hundreds of customers complain about the gadgets.
It said many people could be entitled to refunds as the devices have effectively stopped working.
"Retailers have a responsibility to ensure that products listed on websites or sold in stores are fully functional," said a Which? spokesperson.
"This is especially relevant to products that include a digital component that's vital to operation, which is typically the case with wearables."
Jawbone went into liquidation last year but, as it was wound down, one of its founders, Hosain Rahman, started a new company called Jawbone Health that planned to develop more sophisticated health technology. Many former Jawbone employees went to work at the new company.
Jawbone Health pledged to keep running the online services that owners of Jawbone fitness trackers use to log workouts, track sleep patterns and monitor their health.
In May this year, Jawbone said on Twitter that its old app service, called Up, would be "transitioned" to a Health Hub set up and run by the new company.
Jawbone warned that the transition might mean service disruptions for Up users.
Which? found that instead of just losing access intermittently, the Up service had been "out of action for more than four weeks" and had attracted more than 600 complaints via Twitter and Facebook from angry customers.
The issues with the app rendered Jawbone fitness trackers "useless", said Which?
Despite this, the trackers were still available via three online stores until the last week of June, Which? found. All three, Amazon, Selfridges and GroupOn, have now stopped selling the devices.
Which? said the issues with the app mean owners of Jawbone trackers could be line for refunds as the product is now not fit for purpose.
If the Jawbone was bought in the last 30 days, owners can go back to the retailer where they bought it and ask for a full refund.
Anyone who, in the last six months, got a gadget which has now stopped working can ask the retailer to repair or replace the device. If they cannot do this, said Which?, owners can ask for a full refund.
Anyone owning a Jawbone for longer than six months can ask for a partial refund.