Does anyone know (or have been able to work out) what the actual FB criteria might be for "experienced seller"?
You mention 100-odd FB, but I haven't seen the "experienced seller" banner on any of those sellers' listings that I've happened to look at, who I would consider to be "new sellers". Even with over 500 FB and several years on eBay I don't expect to get an "experienced seller" tag.
I would think the wider the criteria this tag is applied to, the less meaningful it will be to buyers. On the other hand, who would want to see "Novice Seller" on their listings, and who would buy from such a seller, seeing that tag like a billboard warning? I am sure these tags are meant to draw attention to the positive features and assets of the item/seller and encourage purchasing, not deter buyers from buying.