Where did my credit go?

Please help, anyone!

I bought a nokia 6680 with a Vodafone PAYG sim on Ebay a few weeks ago and I had problem topping it up. My credit just went missing after I topped up. The last time I used a Tesco cashpoint with my debit card and checked my credit straight away and it was there all £10 of it. But when I got home it was all gone!!!!! I contacted Vodafone but they asked for the registered name, pin code and password before they'd help me. So I emailed the seller and asked for these but he hasn't replied. I think he's reluctant to help. I have lost nearly £20 just like that, it just misteriously dissappeared.

Anybody has any idea what's going?

Thanks!