I did a Barcelona car rental search on Kayak while on WiFi in Lisbon airport. Kayak passed me off to "Holiday Rentals" which says "OK rent a car" with thousands of positive reviews will charge US$7.42 TOTAL for seven days and include CDW. If it were only the unbelievably low price, I'd suspect a trap. Yet all other signs seem legit. I wonder whether on turn in, they'll say something like "that dent wasn't there before and the repair will be a thousand bucks more than your CDW limit." OR, since they have a shuttle pickup at the airport, could it be a trick to get someone to a remote place for a shake-down?
Youre missing some conditions on the rental.
After a search through kayak, i've seen some offers as low as 9€, and these are the conditions of those given directly by kayak (7 days, 29€ total the lowest):
Pay online and get a 75% off ( weird offer, but acceptable and legit-esque ).
Total does not include fuel charges, car deposit must be returned full.
KM policy : 0.157 EUR per KM, with a maxium total of 1600km.
Rental Insurance : you need to have 1110.0€ on your account, that will be locked to ensure the car's not damaged or stolen, and that will be unlocked and returned on car return.
<25 drivers: drivers between ages 21 and 25 will be charged 5€ daily up to a maximum of 60€.
Additional drivers: any additional driver will cost 5€ daily.
So the real cost will go at around 60-70€ or more given that you'll have to replenish the diposit + 7 days of kilometers.