Scores of shoppers say they've been left out of pocket after buying products from secondhand tech store, CeX.
Some customers, including keyworkers, have reported not receiving their online orders, while others have been sent faulty products.
This comes as more shoppers turn to online retailers for phones, tablets and game consoles during the isolation period.
Which? is calling on CeX to refund all customers who have lost money during the COVID-19 lockdown.
Update on 29 April 2020: CeX has refunded Donna and issued her a £50 goodwill voucher.
Donna Blakey is a mobile carer and keyworker.
In need of a phone with better battery life, she bought a £600 model from the CeX website.
Donna took time off work to ensure she would be home, only for the parcel not to arrive.
'I'm literally desperate for a new phone,' Donna told us. 'I'm visiting vulnerable clients and need a phone to ring ambulances. My current model only lasts for an hour off charge and I can't afford to buy another one.'
More than a week later the tracking status still said the parcel had been dispatched but not yet delivered to the Royal Mail.
CeX told Donna it will look into her case if the parcel still hasn't arrived 14 days after she placed the order.
But for Donna, whose job depends on having a reliable phone, this isn't good enough.
'It's not my problem that they've lost the phone or don't know where it is. I just want my money back,' she said.
Donna ended up making a chargeback claim through her bank, who were able to claw the money back into her account.
CeX told us it regrets courier delays and is following commonly accepted procedures for delayed items in line with courier guidelines.
One customer was disappointed to find the phone they had bought in-store was faulty.
CeX assured the customer they would be able to return the item by post if stores closed due to lockdown restrictions.
However, after stores did close, the customer was told he would have to wait until shops reopened to get his money back.
'It was a blow,' he told us. 'Now is not the time I want to be short of that kind of money. I've got an £80 phone just sat there and I can't return it or use it.'
CeX told us it's unable to take store returns by post and has never offered this service. It assured us it has extensive testing systems in place for all products.
Which? spoke to Chris who opened up a PayPal dispute over his £370 order.
With no sign of his parcel being delivered, Chris looked on CeX's social media channels and found a litany of complaints similar to his.
'It was a bit of a rabbit hole really,' he told us. 'I looked at the complaints from other upset customers and had that sinking feeling in my stomach.'
Chris did manage to secure a refund after he contacted CeX's CEO, but the process was far from easy.
'The complaints procedure is very difficult,' Chris recalled. 'Anybody buying from CeX really needs to know their consumer rights.'
After receiving the refund, Chris' parcel arrived 18 days after he placed the order. It was faulty.
Which? Consumer Rights editor Adam French said: 'While the coronavirus lockdown has impacted online deliveries, it is not acceptable for any retailer to be withholding refunds for faulty purchases or late deliveries.
'Customers have 14 days to cancel most online orders that fail to arrive on time and are entitled to a refund, repair or replacement if their purchase is faulty.
'If you are struggling to get a refund from the retailer, then it is worth contacting your bank to make a claim either under Section 75 or chargeback, depending on how you paid.'
Here's what you can do if you're a CeX customer left out of pocket.
If your product is faulty, you have 30 days to reject the item and ask for a refund, repair or replacement.
And if you're outside the 30-day window, you have to give the retailer an opportunity to repair or replace the item before asking for a refund.
Under the Consumer Contracts Regulations you can cancel orders for most goods bought online the moment you place your order until 14 days from the day you receive your goods, and get a full refund.
If you paid for special delivery, and the parcel doesn't arrive in the agreed timeframe, you're entitled to get this money back too.
Some couriers are running delayed services at the moment due to the COVID-19 outbreak, so it's worth bearing this in mind when placing orders.
If you paid by credit or debit card, you may also have extra protection.
If you raise a PayPal dispute, don't be pressured into closing it.