Listia uses a proxy bidding system, which means the system only bids as much as it needs to in order for you to be the top bidder, up to the limit you placed.
For example - another user places a bid of 5 credits on a listing you want. If the listing starts at 0 credits, the top bid will now be 1 credit since that's all it takes for the other user to be the highest bidder. If you now place a bid for 3 credits, the other user would still be the top bidder at 4 credits because their maximum was set to 5. If you placed another bid for 10 credits, you would be the highest bidder at 6 credits since that's all it takes for you to become the highest bidder.
Placing a bid early can have great advantages. You can set the most you'd want to bid on an item so you don't bid more than you feel it's worth. If it goes over that bid, then you know it's being overpriced. It also helps when listings result in a tie. If the listing ends and two users bid the same amount, whoever placed the bid first will be the winner.
When you bid on an item, the listing holds all of the credits you used until you're outbid or the listing ends, so planning when to bid is important. It's possible another item you want to bid on is ending soon, but your credits are being held by other listings.