Like another poster has said, it bids in intervals.
He also said, pick your final amount and stick with it. If you are willing to pay 300, just bid it, walk away and check later if you won. Don't let emotions affect your judgement. I agree 100%.
Never bid from anyone who doesn't have at least 10 transactions (my opinion) with good feedback. Some good people have less than 100% feedback because you can't please everyone, but be sure to read their profile.
Anyone with no feedback selling might be a fan, but is more likely a scapler/trying to make money ( especially if their handle is oddly U2 related).
However, that doesnt' mean they aren't selling you the geniune tickets. I'm sure some jerks make fakes, but at least most people on eBay are selling real tickets.
I would only pay via paypal and use a credit card (so you can have protection later). I don't know if using a debit card offers the same protection or not. and I think paypal itself has some built in protection.
I bought tickets for Elevation from yahoo from an obvious ticketguy as his handle was great8ticets or something like that. I knew he was a scapler, but he had a decent price and all his feedback was on good sales for other shows. So I trusted him. It was for 2 GA to a Hartford, CT show. It was 125 bucks a ticket and it was the best show I ever saw. Worth every prenny.
Another good tip that I recommend to all is to be sure to write an e-mail to a seller with any and all questions. Judge their answers and response time.
Sometimes I ask questions I know just to see if they know. It works at retail stores too. YOu can tell who is a fan/scalper by what they write (not always of course).
And if they don't respond to you before the auction ends, don't bid. That means it will be hard to contact them later if you need too.
Also, click on the link that shows what else they are selling. If they are selling five shows at once, but claim they "changed plans" then they are lying.
So I'd say 90% of the time if you buy from eBay, you get what you wanted. You just have to use your instincts/intellegence that all U2 fans have, to protect yourself from the other 10%.
Good Luck.