503 Unavailable on Safari Works on Chrome

Hi,

Whenever I navigate to certain website using Safari on Mavericks (15" MBP 2.3 GHZ i7) I get

503 Service Unavailable

However, if I use Chrome it works or if I use Hotspot sheild to mask my true IP address it works.

I has been happen for at least a week on ebay.co.uk and I have just also noticed it on audible.co.uk.

I have tried clearing the cache, removing preferences, getting rid of cookies etc.

Any thoughts?

Thanks.