Here's an intriguing question that just occurred in a game where Haggler, Farmland and Border Village were all in the kingdom.
I had a Haggler in play. I bought Farmland.
Now I have two side-effects I can perform: trash-and-gain with Farmland and gain-a-cheaper-non-Victory with Haggler. By my understanding of the rules, I may perform these in either order, my choice? So far so good.
I choose to take the Farmland on-buy trigger first. I trash a $4 card and gain Border Village. As a side-effect of gaining Border Village, I may gain a cheaper card.
So now I can gain a cheaper non-Victory from the Haggler's while-in-play effect, and gain any cheaper card from Border Village's on-gain effect. But can I do them in either order? Or, now I've started resolving the Farmland on-buy trigger before the Haggler while-in-play effect, must I completely resolve Farmland first by dealing with the Border Village's on-gain before I may use the Haggler while-in-play effect?