I know what a CoD trade is, even if I'm unsure what it stands for, but how do I start one? Thanks-

Select the item you want to send and look at the row of icons at the bottom left of your inventory, click the one that looks like an envelope with an arrow, the choose who you want to send it to and enter the CoD fee.

Cod means cash on (inseet word starting with d that makes sense, pretty sures its delivery or deposit or something lime that)