You mention MP and I agree with you. I feel the same way about handing over my NINO and there are (tens of?) thousands of other ex, or soon to be ex-ebay private sellers.
I've sold nothing since May, until this week, despite slowly increasing my number of listings all through the year and faffing about at intervals re-listing as "Sell Similar" to try to prevent ebay totally hiding them when it decides they're 'stale'.
I put up with MP, although I preferred PPs better and quicker service. I hate the Horrible Hub and always have done because the "Classic" All Selling page suited me, as a small private seller, perfectly, but I was railroaded onto the Hub anyway.
The mix of removing the old Categories and the (Still incomplete) transition to ebay's bizarre IS based searching system and its even more bizarre search results, virtually ended my sales. Add to that my loss of visibility because I don't Sponsor my listings and ebay has almost finished them off. (Four in ten months this year. Ten years ago I was selling more like four items a week.)
That's just my selection of ebays worst changes, others who have already left will probably have different sets of reasons for doing so. So it's now "Free To Sell" on a site that first crippled, then wiped-out my sales and those of so many others.
The site remains the same, apart from a big new banner over the door, and I wonder how many sellers who left for any of those various reasons will return to bang their heads against the same wall a few more times?
As for the hoped for influx of new sellers, how many will sell anything without knowing that 'sponsoring' is now a prerequisite ?
How many will be shocked to find that their cash will be on Hold because they're New Sellers ?
How many will be disgusted to find that they've been awarded a defect for 'Late Delivery' because they don't know how to begin avoiding them?
How will they avoid having their items stolen by devious buyers who know far better than they how to work ebays MBG to their advantage?
Etc., etc., etc.
Will they come back to sell more than once or twice on a site that might be free to use but is also more complicated, user-unfriendly, stacked against private sellers and generally no longer set up to meet the needs of small private sellers?
I think the days are gone when every disgruntled seller who left was instantly replaced by a dozen others. So many have come and gone, so many giving ebay a "Bad Review" for so many reasons over years, to anyone who cared to listen. So I don't think ebay can rely on private sellers (New or Returning) to get it out of the hole that it's dug for itself. It's used, abused, neglected and generally ignored them for too long and ebay's going to find all those chickens coming home to roost.
