I purchased a proven pair waited and waited never heard any calling or saw any eggs. Months later I put another male in the tank and the "proven pair" tuned out to be two females, now isn't that just amazing?
I paid a pretty penny 5 years ago for a proven pair of azureus from a very reputable breeder. The two frogs were very distinctly different in body shape and they certainly spent many days together in the coco hut and chasing each other around. They just never seemed to produce eggs and only rarely could I hear the male calling.
A few weeks ago I noticed my female was acting strange. I watched her closely and discovered she was calling....

She is a he I guess.
So, before you shell out the big bucks, understand there is a difference between "proven" and "sexed" pairs.
Situations like that are one of the few topics that really get me legitimately pissed in this hobby.
Those are things I would personally give negative vendor feedback for if the seller didn't fix it ASAP (obviously you guys couldn't, just speaking in general). A
proven pair is a
proven pair and stuff like this really irks me.... it's COMPLETELY shady and really is a huge load of BS. Even if the seller did compensate me, I still wouldn't buy anything from that person again and would make it a point to share the story liberally.
Pretty much the only thing that isn't 100% lying is when two frogs are taken from a proven group and sexed- but then they shouldn't be advertised as a proven pair, so it's still utterly weak sauce and still deserves compensation IMO.
Buying a probable pair is a different issue, as there is room for discretion & error..... in "proven pair" cases, it's total dishonesty and bad business practice.
When buying proven pairs, I personally think holding off on feedback until you're convinced they are at least male/female is the way to go. If the seller was truthful, he gets his positive feedback anyways, albeit with a bit of a delay. If he lied, he loses quite a bit of business. It's one of the few ways (along with spreading the news) we can combat indecency and the selfish idiots who feel like they can get away with it.
As sad as it is to say, without incentives for doing right and punishments for doing wrong, there will always be some bad apples out there ready to backstab a fellow hobbyist for a few extra bucks. F*** that, and shame on the fools who pull crap like that.