Vinted have a managed postage system so it can’t be illegal 

however theirs is vastly more inclusive of many couriers and prices are fairly cheap  (the RM ones the same as buying direct though which is still ok)

maybe eBay promised to include more carriers  in the future and that lets them off the hook lol 

No idea 

end of the day if simple delivery works then all is good in the main 

but it isn’t at the moment,  eg for larger heavier parcels  the option doesn’t appear when it should

only reason I can think of is that fixing this is tied to fixing something else and they’re in a pickle

we are in the dark and eBay just say it’s being reviewed (like for 2 months now)