So I've been doing some solo runs on the Iron Serpent gate recently, hitting all the clockworks levels down to D14 twice in a row to farm punkin tokens.
The first time down I checked basil's inventory and grabbed a seraphic mail recipe. The second time down, his recipe list had doubled in length, and the seraphic mail recipe was shown as out of stock. I think basil's inventory didn't get cleared between runs or something.
http://steamcommunity.com/id/nicoya/screenshot/596951791364325719?tab=pu...
You can only purchase 1 of a particular recipe in a day (no idea when it resets). His inventory length is variable as well.