Thanks Kiji! Alcaster's shop is in the next update (not live yet), ported from his original script. One difference from the report: he opens it after the follow-up quest 'Alcaster and the Dark Crystal' (bring him a Dark Crystal and a Star Rock), not right after The Book of Ancient. After that he sells Holy Water 300, All Cure Potion 400, and Magic and Summoning Rocks for 5,000 Vesos each - your prices were spot on. Please re-test once you've finished that quest.

Marking this fixed. The change ships in the next update. If it still happens after that, please reopen this report.