You have to be an active, participating, member of the site to start new topics - especially Buy/Sell listings. This is to prevent abuse, spam and to deter scammers (e.g. those with no post history/reputation and who therefore have nothing to lose by being a bad actor).
That takes a couple of weeks of normal interactions (not just reading, and not just making random/pointless posts on other people’s threads) for the system to promote you to a trust level that can start posts (and thus make Buy/Sell posts).
In other words, just trying to make lots of posts in the hopes of gaming the system will have the opposite effect.