Sniping is one of the auction anomalies created by eBay. No other auction house in the world holds auctions like eBay does... The whole point of an auction is to continue the sale until no one wants to pay more for an item... That being said, here's the thoughts on Sniping:
What it is is placing your max bid in the last seconds of an auction. The idea is to not allow enough time for someone else to get outbid and think 'Damn, I'll pay more for that' and try to increase their bid. In a real auction, this happens all the time. Someone will think they won't pay more than $100 for an item, then when they get outbid at $125, suddenly there's a combination of the excitement of the auction, and thinking 'hell, maybe it's worth more than I thought' or 'Man, I really don't want to let that go' so they bid again... And sometimes again, and again, and again...
It doesn't necessarily happen on every auction, but how many times have you bid your 'max' on something on eBay in the last few seconds, only to get outbid and think 'Damn, I should have bid more'... (And I don't mean just on pins). For a lot of people, their experience with auctions is mostly limited just to eBay, so there's a lot about this 'bidding frenzy' mentality that they sometimes don't understand. Standing on the floor of a live auction (or, these days, watching it live on your computer) and waving your hand in the air amongst a group of exciting people cheering you on does wonders when it comes to re-thinking how much money you can spend... (And, yes, that can kind of be a bad thing... It's like going to Vegas, if you don't have the self control to just cut yourself off at the gambling table when a certain amount is spent, simply DON'T GO! Same with live auctions of things you really like...
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So, to answer your questions:
1- Placing what you believe is your maximum bid amount on an eBay 'auction' in the last seconds of the listing.
2- In hopes of getting as cheaply as possible (not allowing enough time for counter-bids).
3- See #2 and: Also, waiting until the end to place your bids helps reduce the 'popularity' of the item. If a lot of people are bidding throughout the 7-day run of the auction, that will raise the price early on, and people will start thinking it worth more, and raising their max bid, even if they snipe... Sort of a slow-motion auction frenzy.
4- Snipe programs/services are nice because you don't have to be at your computer to place the bid. I use an online sniping service, so I can 'place' my bid with them as soon as I see the auction, and I'm notified if I win when it's over. I can also change (increase/lower) or cancel the bid at the sniping service at any time without any penalties... I just consider it a nice front-end for eBay.
5- Yes, in the sense that it buts down the frantic 'bidding frenzy' that any normal auction has. But if everyone truly has their max bid determined in their head, and they all place it 10 seconds before the end of the auction, who ever is willing to pay the most still wins... (But no one gets that chance to think 'wait, I'll go a few hundred dollars more'... (Trust me, I've been to live auctions... I've spent more than I should at them... It happens to all of us.
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6- I place all my final bids via my snipe tool...
The main drawback? No matter how you bid, if you wait until the last seconds, SOMETHING could go wrong and your bid doesn't get placed. Your internet could go down, the internet of your snipe service could go down, all of eBay could go down... None of it will matter. eBay stance is 'You've had 7 days to bid, you shouldn't have waited until the end'...
eBay really should allow sellers the option to auto-extend auctions by 5 minutes if there's a bid placed in the last 5 minutes... I've never understood why they don't do this (that's what an auction is, after all... Ever heard of a real auction ending without the familiar 'Going... Goooing... GONE!' statement by the auctioneer?) Even Yahoo auctions have this option... But any other online auction service that ends at a specific time got the idea from eBay. They made a major mistake in their auction format software, and now everyone seems to think that's how auction should work...
I've never bid on a DPF auction, so I can't speak for that. But, from what I know about them, since the seller DETERMINES who the highest 'bidder' is, waiting until the last second doesn't really make any sense... You could bid your entire collection in that last second, and the seller might still decide he/she wants the one pin off their Grail list offered by someone else...
I, personally, don't think ANY auction should be forced to and at a certain time. Wait until there's no counter bids for an hour or so, and then announce that you will be closing the bidding window soon (i.e., Going once/twice), and then post that no more bids will be accepted. Then make your winning announcement if it's not already listed via leaderboard or something...