Fausto,
Some members that have a knife up for "sale" may be open to trades for a knife or knives representing commensurate value. Generally they will say so within the text of their post.
It would not be wrong to contact the member privately to ask if they are amenable to a trade if there is a question in your mind or if you happen to know that the member may be interested in a particular knife or knives.
There is also the "Knives For Trade" section of the "Exchange" which is where any member (regardless of membership level) may offer a knife or knives for trade.
Money cannot be involved in the exchange and any violation of the rules would be dealt with quickly and harshly by Spark and the moderator staff for obvious reasons.
At this juncture the "trade" section does not provide for different categories and sub-classes of knives.
I have discussed the option with Spark to have "traditional knife" oriented categories in the various exchange venues and it was determined that the current area is the one that needed to be addressed most immediately. Spark left the door open for us to revisit additional changes as we go forward. Obviously it makes a certain amount of sense that those supporting the site financially would have their needs and concerns addressed as a higher priority.
(This is not meant to convey that all members of any given membership level aren't equally valued for their contributions to this site.)
If you need further assistance feel free to reply here or to contact myself or Gus directly. I hope this helps answer your questions.
Edit To Add:
Thanks to all for the good words on behalf of myself, Gus and Spark. :thumbup: