For a limited time, you can initiate a local trade offer (for buying or selling) from the local chat menu. EDIT, later: The experiment is now over, so if you are reading these words, you won't be able to use the trade offer feature any more (unless you are reading this way in the future when we turn it back on).
Anyone in the same location will see what you want to buy or sell and for how much with a little talk bubble over your head, similar to how race offers work now. Clicking it will open the trading UI with the offer options pre-set so you can negotiate from there.
We're testing this for a few hours only right now to get some early feedback (and maybe even find some bugs). When we release this properly, the feature will probably be restricted to specific marketplace locations so trade offers don't get too spammy in the rest of the world.