So I built a trade depot and foreigners come with goods to sell. I designated some trade items, and offer a hefty sum (say 150% value). Nothing happens. I get a message about an absence of goods on my side. What do I need to do to trade? Are my people supposed to move designated trade goods to the trade depot in advance?
No, they just need to be trade designated (green). The swap is instantaneous. Was the message something about our goods have disappeared? I think that one happens if you take too long and the traders depart; using space to pause while setting a trade up may be a good idea.
It only needs to be an even trade If you designate a trade good and then it gets used, eaten etc, then you'll have problems. Usually I only have a few less-used items for trade (mostly glass bottles)