The vendor has nothing to sell till you hit 400 so why not fill in the almost useless favor levels of 100 and 250 with things like the Mark for Crafting, Silver Flame Heal Light, Moderate, Serious, and Critical, that have maxed Caster level. E.g. SFHMW potion would heal both WF and Fleshbags for 2d8+10, similar to how Lily extracts used to work. Could also add more expensive pots for things like Restore and FoM as well as Single charge exclusive raise dead Gems similar to the Planar Shard turn ins to the 400 favor mark to make it more appealing to buy these packs and to farm this favor.
Right now there is very little reason to farm out SF pots, as they have many down sides and you have to do nearly every SF quest on Elite. Most of these quests are some of the most annoying ones in the game, whether because of quest design and mechanics (just so you know I love almost every quest in all 4 Necro Series) or because of bugs such as disappearing GS, making it nearly impossible to find a group to do Elites with. Giving tangible rewards along the way as well as rewards that are useful to all characters, would make both at level elites and the packs themselves much more appealing.
/EDIT
Disconnected thoughts reconnected