Does lying indicate that the trade risk is higher than that of someone who is honest, thus requiring negative trust?
IMO, it depends on the kind of lie that was told. I really cannot think of or describe any of anything now, but there are some kinds of heavy lies that are causes loss to a merchant, and such lies deserve a neg tag since it's going to be a risky trade than for an honest person.
Are lies an indicator of trade risk and do they warrant negative trust feedback?
I think it's not all lies that just deserve a negative tag, unless you are really aware that it would be very risky to trade with a person who is not straight-forward in their behavior.
Sub-topic:
How different would the forum be if all neutral feedback toward dishonest and lying members was changed to negative?
To some extent, I think that a neutral tag is just like an alarm trying to warn people to be careful if trading with such a person, but a negative tag is a clear warning for other members to avoid trading with such a person. So, depending on what the offense is, that will determine whether it warrants the neutral or negative tag.