Buyers vs. Sellers

I don’t understand why teams have to be “buyers or sellers” in the trade market. Why can’t teams assess one another’s needs and try to fill them? For example, the Mets have surplus starting pitching, albeit a pretty vanilla assortment after Pedro. Anaheim thinks that Darrin Erstad is their best first base option, leaving Casey Kotchman out of a job. Since they lost out on Carlos Delgado, the Mets have suffered through Doug Mientkiewicz war against OBP. Meanwhile, on the left coast, the Angels have had good things from Bartolo Colon, Jarrod Washburn, Paul Byrd and John Lackey, but need another starter for an AL West that’s starting to look competitive. Escobar can’t be counted on for much, and Santana has been below replacement level so far. When Steve Trachsel comes back from the DL, he will join Martinez, Benson, Zambrano, Ishii, Glavine and Heilman as (debatably) capable starters. Seven starters, no first basemen. There seems to be no need for either of these teams to be “sellers” to help one another out. Would it really be such blasphemous Post-fodder to swap out Heilman and a prospect for Kotchman’s bat?