To get the Black Friday achievement in State of Decay 2 do the following:
Other survivors in the world not belonging to your settlement can almost always be traded with, unless they're busy with an action or hostile toward you. Approach them to talk and from the options choose "Trade with Enclave." Trade them ten items, for which you'll gain Influence in return, and this will unlock.
If you have enough Influence, you can even sell and buy the same item back and forth ten times each to also unlock the following achievement: