<p>I mean, can you use couchDB for:</p>

<ul>
<li>CRUD of items, users</li>
<li>bids and auctions resolutions</li>
<li>bidirectional califications</li>
<li>forum</li>
<li>items comparison</li>
</ul>

You could try to use CouchDB for an application - as to whether you would be successful is another question.
Something on the scale of eBay will have special requirements that are not representative of a typical application, If you are building a small auction site then perhaps CouchDB would suffice. A document-oriented database like CouchDB may not be so hot when you have to deal with transactional/records-based data like that associated with auctions.