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For me recently, buying and then getting lost in the system. The one time I sold went fine. Apparently they've upgraded the system which has teething troubles, (but that doesn't explain all the recent issues).
One I bought and returned a year ago - When you order a lens described as with a smooth focussing ring and with a lens hood, and when received it has gritty focussing and no lens hood, you have to wonder what's going on and why they bothered to post it the in the first place...
All resolved now though so /rant
Partly for the wonderful photographers here and partly for catharsis, MPB have been consistently shite across the board in 75% of my transactions. Other options are available.