If you hold an item that the villager is buying, it will hold out an item that its selling corresponding to the item you were holding. Now if that item were to be a totem of undying the villager holds, you can kill it and make it use it to glitch out that trade. I would expect a different result like either the trade being removed or the totem still being able to be traded, but it just transforms the totem item into air in the trade.

To replicate this, summon a villager that is selling a totem for any item, using the command below, and hold out the item it's selling, like the emerald using the command, then kill the villager and watch it use the totem. Now you can open its trading menu and see that the totem item has been replaced with air. I wouldn't expect it to be like that and would expect a different outcome or even having the villager not able to use the totem. But I guess this is what happens if you perform these very specific steps.

/summon minecraft:villager ~ ~ ~ {VillagerData:

{type:plains,profession:farmer,level:2}

,Offers:{Recipes:[{buy:

{id:emerald,Count:1}

,sell:{id:totem_of_undying,Count:1}}]},Health:1,Attributes:[\{Name:"generic.max_health",Base:1f}]}