Waiting until the last moment to place your bid on an auction may work well for you. 

However if you're bidding on a fixed price item and you wait until the last minute to send your best offer  the seller can't accept your offer even if he, or she, wanted to.  The minute the time has expired, the listing expires.  So to accept your offer the seller has to pay more listing fees, etc., and take the time to relist the item.  I've had this happen on my listings more than once and I never sold the item not withstanding a few of the offers I would have accepted.  So if you wait until the last minute to place an offer on a listing you may as well not watch the item and forget about it, its not going to happen.

So if you see something you want go ahead and send the seller an offer. No one is going to see what you offered, and you're not going to see what someone else offered.  At the very least send the offer within 24 hours of the auction ending to give the seller the time to receive and reply to it.

Happy buying and selling :-)  Please take the time to indicate if this information has been helpful to you. Thanks, I appreciate it :)