Richard Polhill wrote:

It's pretty clear that the lock on the aperture ring mechanism is
faulty, and gets stuck just before f11. It can be moved past this by
holding down the lock button (though I can hear a metallic click as it
goes past).

The lock button should stay down until the ring is in the A position,
but it rises after f8. What are the chances of having this repaired at a
reasonable cost, or even of doing it myself?

Personally I'd have a go myself, but you might not be as keen to take a lens
apart. The cost of repair would outweigh the cost of replacing it from eBay.

I repaired a fairly similar Praktica lens, but I had less to lose as
that was non-functional - this lens is otherwise perfect, and works
beautifully in all other respects.

If there were some way to get at the lock mechanism without messing
about with the focus screw, I'd certainly go for it.

Did the eBay listing say it was faulty? If not then you are entitled to return
it and get a refund.

I'm in touch with the seller, but exploring options.

Daniele