I think your answer lies in Talent text you didn't quote.
The adept drives a “hard bargain” when buying or selling goods by making a Haggle test against the customer’s or merchant’s Social Defense.
Haggle isn't contested, it's versus SD. There can be back and forth as each participant attempts Haggle, but the critical thing is it is not contested.
Why is this critical? If it were contested, then there is always one failure on every Haggle test, meaning the Talent/skill could only ever be used once.
However, it's not contested. When someone "loses," it's just because the Haggle test was successful in that result beat his/her SD. So, it could go back and forth multiple times before someone fails to beat the other's SD. At that point, a Haggle test has failed, and all haggling stops.
The end mechanic is kinda weird. Basically, someone with HORRIBLE Haggle (is it a default skill?) can immediately shut down pricing discussion on purpose just by making their terrible roll out of the gate. The result is that a total Haggling God looks at his Rank 15 Haggle, sighs, and says "ok, Standard price it is." From a perspective of a player doing this, it falls under one of the two fundamental principles every table should live by: "don't be a d*ck." From a NPC perspective, its problematic. If a PC attempts to Haggle, it only makes sense for the NPC to try as well, even if the NPC sucks at it. So, what could've ended up with an awesome 50% discount with your PC's Rank 10 in Haggle stops at 5% because the inept counter-proposal from the vendor fails.
It makes more sense to me that each side can Haggle until THEY fail, at which point THEY cannot Haggle further. Haggle Rank 10 guy can then still get the potential 50% change regardless of the ineptitude of the vendor.