This isn't a hard achievement, just have to play enough to get 10000$ worth of items you are willing to sell.
What I did was sell all my extra mods, to do so, go to your mod station, click on Sell, then push in your Left Stick, this will sell all your duplicate mods. If you don't want any of your mods or just really want this achievement fast, Click A on all your mods you see then hit X to sell all.
You can also sell things by going into your equipment when you press the start button, then inventory to see all items. Anything with 3 blue boxes or papers next to a number you can sell for credits toward this achievement.
Do note anything you sell you cannot get back so easily.