On the black market menu, look under all the stuff about buying and selling. Mouse over a few (i think its the little one with 5.0 on, bottom right) and click it. Then you can sell metal and crystal to allies!
Hope this is actually right?
mostly. but I think you don't actually sell to allies per se, but to the market. when someone else buys that resource over the black market, the black market 'stock' fills up and takes your offer into consideration which gives you money. it should also influence price.
if you don't need cash immediately and find yourself in structural metal or crystal overproduction, this is a VERY useful feature. at least if there is enough demand for the resource. just plan it accordingly so that you don't suddenly find yourself needing cash and having resources available. place them regularly and constantly and you should be fine. and get the achievement. make a big, long game for that one.