Is it legal to sell secondhand CDs and DVDs online? Yes
Is it legal to sell secondhand software online? Yes, as long as the software has been uninstalled from the seller's PC.
Is that site legal? Probably not, because it's a criminal offence to trade on-line without including a valid postal address on the website. (If that address appears when the purchaser reaches the checkout page, that would be OK but it's impossible to get near this page because the 'Add to Cart' buttons don't work!).
Chris