40d:Trade depot

Building a trade depot will allow you the opportunity to trade with caravans that arrive at your fortress. Each of the three friendly races will send a caravan once per year, however they will not arrive unless your region is marked as accessible by them. Dwarves will always arrive regardless of location, however.

Human wagons on top of the regular mules will arrive if there is a minimum 3-wide path from your depot to an map edge. You do not need to have a road, but you will need to designate smoothing and tree-cutting along the path. Use D to verify that there is a wagon accessible path to the depot.

A broker is not required but is preferable for getting better deals. To toggle wheter anyone can trade or just the broker is done with b. requesting a trader at the trade depot will queue a job on the traders list. A higher level of brokerage will give you greater returns, so if you're dependent on trade it's recommended to keep one dwarf as broker and level up his appraisal skill as high as possible.

Dwarves

The dwarves have no issues with hours and hours of bargaining until you get the trade just right. Occasionally, they'll make counter-offers for you to consider.

The merchant will meet with your liaison and discuss what you would like to buy next year, as well as suggest things to sell. You may buy from the dwarves pretty much anything you could have brought with you during setup, except for pets.

Elves

Elves bring you wood and plant items, as well as caged tame pets. However, they will not accept wooden or wood-derived items in exchange.

In addition to the previous restrictions on no dead things, apparently you have a limited number of attempts at a trade they like (my novice broker had three or four) or they'll just pack up and leave. Apparently a pile of stone crafts was not enough to buy all their seeds off of them (message: "I can't let you end up with ALL of that").

 Note: Sometimes when you get that message then pressing t again will make the trade regardless. 

It is presumed that this happens with other races as well and that how many attempts you get is influenced by your Comedy/Negotiate/Intimidate/Etc. skills of your broker.^[Verify]