They look to be legit enough, they've been an ebay member for over 10 years, are based in the UK (unlike a lot of sellers that say the item is delivered from UK) and have a lot of good feedback for some high value items. I did notice however, they ask for payment by Paypal or bank transfer and common sense would advise you pay by Paypal until you get to know a seller rather than jump straight on to a BT. Panamoz have the 24-105 at £503 or £477.85 if paid by BT, but with Panamoz you can find a lot of good reviews on here from very satisfied customers. I've bought from Panamoz and fully trust them enough to pay by BT (I bought a 24-70 f2.8L II by BT only a couple of weeks ago) and to recommend others to do the same.
However, it's your money and you obviouly want to use it in the best way possible and you can save a few pounds by buying from the ebay seller, in all honesty I doubt there would be any problems with an ebay seller with 99.9% positive feedback with over 13,000 ratings. I'd certainly give them a look the next time I'm looking to buy something off ebay. Hope you get sorted soon.