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By placing a bid any earlier than the last few seconds you risk the chance of being sucked into an unnecessary bidding war.
And you tip people off that there is more interest in the item. They might up their max bid in order not to lose it, which they might not have done if it doesn't look like there's that many people interested in it. Basically, there's nothing to gain by 'showing your hand', so to speak.
The optimal eBay strategy is to put the maximum you'd be prepared to pay for the item as close to the deadline as possible.
If it goes for more than that then it went for more you were prepared to pay for it, otherwise you should win it and pay up to the limit of what you were prepared to pay.
eBay relies upon people getting sucked in to bidding more for an item to avoid losing the auction, even if this means people end up paying more than they wanted to. By placing a bid any earlier than the last few seconds you risk the chance of being sucked into an unnecessary bidding war.