For the first question
As pointed out in the comments, the first part of your question is a duplicate of this question.
For the Second question
There are a few options I can think of to achieve what you are looking for.
Touch of Truth Telling
Touch of Truthtelling acts as a Zone of Truth but only has a single target and the spell has a very visible reaction.
The target momentarily takes on the semblance of a being of perfect order, like an archon or inevitable, so that all who can see the target know it is affected by the spell.
Both parties in a negotiation would need to have the spell cast on them, but it is a first-level spell so the cost is minimal.
Mark of Justice
Mark of Justice is a spell which allows the caster to designate an undesireable action, and if that action is taken a curse comes into effect. If the triggering action was "lying to the caster during this negotiation" or something similar it could reveal a lie. It only works on someone willing to take the 10 minutes to do it. But it would give more dire consequences to a betrayal.
Specifically for trade negotiations, Fairness forces those involved to trade fairly. Since its target is creatures/level instead of an area, the caster would know if their target saved against the spell.