Re: discovering your high bid
Sure, anyone can do it – they just can’t do it very often without risking getting caught. When the bid is $18.00, they can simply bid $1850(.00). Due to eBay’s proxy bidding system, they will be the hign bidder at just over your highest competing bid (e.g. 45.00) Then they can retract the bid, saying that they left out the decimal point. Lame, yes, but I’m sure enough people genuinely mistype without noticing that it’ll seem plausible to eBay - if they don’t do it very often (There’s an eBay policy on bid retration, but the last time I saw it, it wasn’t very specific on the threshold. I suspect it depends on how many people lodge a complaint about you retracting, and how energetic the employee handling each new complaint is feeling.)
If a bid is retracted, the high bid should go back to what it was before. I’ve seen a few cases where it didn’t seem to, but that was years ago, and I assume that bug was fixed. It didn’t go back down due to a quirk of eBay proxy bidding: you couldn’t just raise your existing proxy bid, you could only enter a higher proxy bid that competed with your earlier proxy bid (a bad idea in most cases).
If the bidding returns to the pre-retraction state as it should, a retracted ludicrous bid would discover your high bid, and could bid just under it (e.g. $44.99) forcing you to your highest approved proxy bid. It is actually expected that you will place a “real” bid after you retract; not doing so makes your retracted bid look suspicious.
In the few auctions where I’ve seen retrations (I’ve never actually retracted a bid myself), it seemed to be a manual practice that required eBay intervention, and probably takes more time to process. However, if you weren’t watching the auction “live” but came back an hour later, after the retraction was processed, the “bid history” transcript might have been “fixed” as part of the manual resolution process, and you might be looking at what looks like a miraculous guess
Or the bid history might show the retration. It depends on how their CSR’s "retraction screen is currently set up, and how diligent the CSR is about policy.
However, it is still more likely that the other bidder’s estimation of the item’s value was very close to yours. In that case, one of you will edge the other out by a hair.
RE: bidding increments
Though you can only outbid the CURRENT high bid by the minimum bid increment, you can outbid the FINAL bid by less, if you set your maximum proxy bid to an irregular amount, or if some other bidder does.
e.g. if the current bid is $50.00, you can’t bid $50.01, but if you set your proxy to $50.01 when the current bid was $45, you’ll beat someone who bid $50. Also if there was a bidder whose LOSING maximum was 48.01, all subsequent bids will increment from that maximum (i.e. 47.00 … 47.50 … 48.00 … 48.01 … 48.51 …).
You might expect the “high bid” to leapfrog over the irregular amount, since your next minimum bid is higher than it anyway, but if you think about it, the most computationally efficient method to match two proxy bids is to compare them once, and se the high bid accordingly, rather than enact the step-by-step incrementation – that made even more sense with the slower, less powerful servers of the 90s, but it still sensible with today’s servers, since millions of bids are made every day, and most of the heavy action is in the final minute or so.