I dunno how I feel about once-per-turn on a Way; just because it's different than all existing Ways. It feels like a weird exception to the normal rule for Ways.. "When you play an action, you can choose to follow the instructions on a Way instead of following the instructions on the card. Unless that Way says once per turn and you've already used it this turn".
You could get around it with the "if you haven't yet this turn" language like Fortune.
But I could be completely wrong; it's not actually different from the exception that exists on once-per-turn Events. Recommend saying "differently-named" though; to prevent just the silly "free" action you could take for no reason every single turn.
The obvious way to get rid of the weird once per turn clause, whose only purpose is to prevent loops, is to make the Way nonterminal.
It wasn't actually to prevent loops* - it was to prevent (outside of on buy / gain effects) just buying any card that costs $x. i.e. if I could always exchange it, then when I have $5 I don't really need to be picky about which $5 action I buy. Sure it becomes a concern later as piles empty, but definitely not early. Also, I wanted to avoid something like getting several peddlers and being able to exchange all of them for Provinces in one turn. While that is also avoidable by specifying "non-Victory" I did want you to be able to use it for 1 Victory.
* this is a side benefit, of course; which is why I didn't worry about adding "differently named" and opted for less text. I guess it could make a difference with Conspirator - does anything else care about how many actions you've played?
I didn't consider "If you haven't yet this turn", that would work too. I think that's still different than all official Ways, as it usually has the same effect*, so not sure we get away from that. I actually have a few "once per turn" Ways I'm in process of designing, so I *think* I'm ok with it as a new sub mechanic.
* with +Actions it would let you play actions you may not want to play that turn (e.g. Forager, Death Cart Scholar) just to play them, for Peddler.
I also considered making it terminal, but wanted you to be able to play the exchanged card without needing +Actions from before.