Do you mean 'offers'? There is a limit of 3 on those, but there is certainly no limit to the number of bids you can make in an auction. So if it is an auction then by all means, bid again.
The only reason I can think of that you would be prevented from doing so is if the seller blocked you because you are brand new and have zero feedback. Most non-payers (people who win auctions and don't pay for the item) are newbies with 0 feedback because they do not yet fully appreciate the seriousness of bidding and winning on ebay. So sellers often remove bids from these types of user IDs and/or block them. A seller can remove any bid for any reason.
If, however, you refer to a listing which invites offers, those have a limit of 3 offers and this is because if you cannot be reasonable in your suggestion by that time it is assumed you are not serious about buying it.
When you make an offer the seller can accept (whereby you go to Purchases and pay), decline, ignore, or make a counter offer. If it is a counter, you can then accept (and pay) decline, ignore or make a further counter. Most sellers do not haggle as a hobby so if you make an offer and receive a counter, you can assume that is the lowest the seller is willing to go. After all, there's always another bidder and they know it.
If you would like to see 'offers explained' ebay-style, there is the following link:
http://pages.ebay.ca/help/buy/best-offer.html
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