collect the free handful first
Talk to the Tavernkeep and pick the crystal dialogue option once and he gives you a starter pile for nothing. It is once per character rather than once per world, so do it on every new character before you plan any spending. In multiplayer the crystal pays everyone separately at the end of a wave, so there is nothing to argue over.
what to spend them on
Sentries are the reason to save. Buy the tier you can actually deploy rather than the best looking entry on the list, since the higher tiers are gated behind bosses you may not have beaten yet. His armor is a summoner detour worth taking only if you are committing to the class. Everything else can wait for a later run of the event, and the full list is a very long grind if you want all of it.
turning medals into coins
Medal prices never move, but the coin value you get selling something back to him does, and that gap is exploitable. Buy sentries, sell them straight back for gold. Make him as miserable as possible first, evict him or dump him somewhere he hates, because the sell price tracks his coin pricing while the medal cost ignores happiness entirely.