Researchers at the consumer watchdog obtained quotes for a number of machines from various online companies and found that those wanting to get a good price for their old computers can recoup up to five times as much by selling them privately.
Investigators took five used laptops of varying ages and ran them through a selection of buying companies, then compared the offers with what the laptops had recently fetched on eBay.
One offered just £67 for a 2009 13-inch Apple MacBook that could have been sold on eBay for £350, while a Dell Studio 15 laptop which would fetch anything between £151 and £215 on eBay, was given a price of only £55.
Which? computing editor Sarah Kidner said: "People should be aware that the amount offered by these companies is not always the final amount you receive.
"Selling an old laptop online can be an easy way to recoup some cash, but it pays to shop around and try online auction websites to find the best deal."
Researchers took three "broken" laptops and desktops to branches of PC World and Currys around the UK to test their newly-revamped PC repair service, KnowHow. Which? said its team had created a simple software error on the laptops relating to the hard drive and had similarly disconnected the desktop PC's hard drive and DVD drives.
They found the KnowHow teams failed to repair a single laptop and only satisfactorily fixed two of the desktop PCs. The investigation also found that timings varied, with one PC being ready within hours and one laptop being held for 19 days.
A spokeswoman for PC World and Currys said: "We successfully repair hundreds of thousands of laptops, desktops and TVs every year - averaging out to about one per minute - and we are constantly working to improve our services, so it is very disappointing to read about the experience of Which? magazine."
© 2012 Press Association