How can I raise my existing bid? by Tyler Jones
Here is my canned response on Raising an existing Proxy or Max Bid. If you need more information, please let me know, giving details about the problem at hand.

If you are the high bidder, you only need to bid again to raise your Proxy Bid. eBay will not allow you to enter an amount that is equal to or less than your current proxy bid, nor an amount that is less than the current required bid. On that last one, I am about 99.999% sure that I am correct.

eBay is smart enough to realize what you are doing. You do not bid against yourself and when you enter a new bid, only your proxy is raised. The current bid will not increase, assuming you are already the high bidder. If you are not the high bidder, then the current bid increases in accordance with various increment and Reserve Price rules.

There are three exceptions to this.

1. If your proxy is equal to the proxy of the second highest bidder and you are the high bidder because you placed the high bid first. If you place another bid to raise your maximum, you will lose your favored "early bird" status. As a result of placing another later bid, eBay will increase your bid to one bid increment more just so that you will remain the high bidder.

2. if your proxy bid is greater than the proxy bid of the second-highest bidder by less than one full increment. In this case, bidding again will cause the current bid to rise until you are leading the second-highest bidder by exactly one increment.

3. If you are bidding on a reserve price auction, your first bid didn't meet the reserve and your second bid did. Anytime a bid meets or exceeds the reserve price, regardless of the presence (or lack thereof) of other bidders, the current price immediately increases to equal the reserve price and the screen will then say "reserve met".

4. If you are bidding on a Dutch Auction, and either bid on all of the items, or you increase the number of items that you are bidding on in such a way as to bump somebody off of the qualified bidders list.