The Buy/Sell/Trade forum is restricted to well-established community members in good standing. The forum automatically grants access to those members that have a certain (intentionally unspecified) combination of number of posts, time as a member and reputation level.
These restrictions were put in place with the intention of reducing the number and frequency of fraudulent transactions, as the forum was really only intended to allow our forum regulars a place to trade goods - not as a retail channel for those whose sole purpose for participation was to peddle their wares. Toward that end, the restrictions have been very successful - in fact - I can't remember the last time I heard of someone being scammed in our Buy/Sell/Trade forum.
There are many other channels for trading goods on the Internet, besides Craigslist. My suggestion would be to seek out other options. Otherwise, continue participating in a positive way, and I would imagine your standing will improve in short order and you'll be granted access.