Just a few years ago, I don't think any of us would have imagined eBay would introduce a postage system where we would have this very limited choice of who we can post with, & how we dispatch items... and that eBay would also be setting the prices!
Many sellers will now be excluded from selling, for one reason or another - and many more will find the whole thing too complicated & stressful to bother with.
Given the choice, the vast majority of sellers would much prefer to sort out their own postage arrangements. It really does look like eBay don't want small, amateur sellers any more, because the BPF & Simple Delivery seem to have been specifically designed to discourage them.
