Cool! Yeah, those were the lines I was slightly confused by. For instance: Do I need to pick a common telltale (say, color), and then pick the rare telltale too (which is “It has no telltale”)? Or: Does the poison need to have a Nasty effect in addition to the [Rare] Deadly effect? If they pick a Rare option for a list, they don’t also need to pick a common item for that list, it seems to me.
If that interpretation is the same as what you intended, I might change the wording to something like this (although it can probably be improved):
Pick one common option from each list. Then, any number of times, you can spend 1 XP to upgrade one common option to a [Rare] option from the same list, or to add a [Rare] option from any list.
There might be a better way to say it. But, some way to say, you don’t need to have a common option in every list, if you have a [Rare] option in that list.