Sable

The best way to make a couple grand quick is to farm Lighting Crystals at Crystal Plateau in the far SW corner of the map. Grab the quest Lighting in a Bottle from the Machinist Zahra at Seven Sisters Station in the northern part of Hakoa (SW region) first.
I'm unsure if these crystals grow back over time but they sell for 100 cuts a piece. Make sure to keep 3 for the quest.

To sell items just ask a merchant to trade then press to get to the Sell screen.
Collect random butterflies, beetles, fruits, etc as you travel to sell them also.

The quickest way to spend 1000 credits is to go to Eccria to find Bike Merchant Ahvid. Fast travel to Eccria in Redsee (NW region) and he'll be off the platform to your right. He sells high priced bike parts and some clothing.