I've been doing a fair bit of buying/selling lately (at least 5 trades per week, every week for the last 2 months) and I have 2 comments.
1) I prefer SKF related trading to Facebook groups/Gumtree because the people here are generally a LOT nicer and less obnoxious/suspicious. But there just isn't that much available most of the time.
2) I admit that while I've got stuff to sell, I've posted it on Gumtree rather than here. At first it was because I hadn't bought the subscription and couldn't post, but right now, it's mostly through sheer laziness.
For my part, I promise I'll remedy that laziness soon and start posting my stuff here. Hopefully others will as well.