Hello 'valley...',
<<Someone told me I can cancel my order and he opened a case for unpaid item but he told me it was ok?>>
Who is the 'someone' and who is the 'he' and are they both the same person, ie, the seller?
And what do you mean he said 'it was ok', -- what was 'ok'?
<< what should I do? >>
I think the real question here is What should you have done?
You should have placed the item in your Watch List by using the link directly under the Bid or Buy button. Then, you could have kept track of the listing whilst price comparing and making up your mind about it. You should have waited until you were absolutely committed to paying for the item before buying it. And then once you bought it, you should have paid immediately.
Ah well, that's all academic, now, isn't it. 
Your choices now are pay up or get a strike. If you pay and receive the item, you can leave feedback.
If you pay and do not receive the item, you can file a claim for Item Not Received and get a refund.
If you do not pay you will get a strike. You will not be able to leave feedback. If the seller tries to leave some nasty remark for you in feedback, you simply call ebay and get it removed because that is a policy violation.
If you get another Unpaid strite it may be difficult to find 'open' sellers as most sellers set up their listings to block non-payers.
So what you should do is entirely up to you, but in future you should certainly put items in your Watch List until you are completely ready to pay for them and never commit to buy anything until you are.
Best of luck.
