Hi -
Big fan of Opera speakers. I still have a pair of Opera Platea's.
The Quinta SEs were/ are a USD 6K speaker new. Opera speakers come with a 5 year warranty, so would these speakers still be covered under the warranty, assuming that they are only 2 or 3 years old? The seller should be able to check with the dealer he purchased the speakers from.
Audiogon offers a blue book service which can guide you as to what the speaker's used value would be.
That aside, what are $6K speakers that are a few years old worth? Opera speakers don't usually come up for sale used that I have seen, but I don't know if that is because people don't part with them or they are not that popular.
Something else to factor in is that this will most likely always be a local sale for the seller given the speakers size & weight. That is worth something, as shipping would be in the $200 or so range.
Personally, I would be south of 50% list, but I prefer to buy new when I can, so it would need to be something very special for me to buy used.
Good luck,
Rich