I've only purchased through Modular Grid, never sold, but I have sold quite a few items elsewhere. My advice would be to list things at a fair price, accurately describe EVERYTHING, use PayPal goods and services for everyone's protection, and ship quickly with sturdy packaging. I know a lot of folks like PayPal friends and family, but I'm not sure I would be comfortable using that with someone who has minimal feedback.
Have fun and good luck.