Summer Trading

Well the top priority should be the crystallized lore after that i would just go after the materials your servants need to ascend and raise their skills.

Hearts, Medals, Bones, Claws, Wings

Depending on your servants, I say what you need most urgently should be prioritised first, then if you have time, grind for the other items.

It depends on what you have, but Hearts and Dragon Reverse Scales (shop of part 1) are among the rarest of the rarest.

And almost everybody (from David and Euryale to Carmilla and Herc) needs at least some of them, so go for it.

After that, Claws of Chaos, that is, if you have servants needing some. Those Chimeras don't spawn that much in CG Berserker 30 and 40 AP as much they should, and sometimes they give you just zerk yellow gems (all my hate...).

If you have a good caster, you will probably need the Forbidden Pages, either for ascension or skills. Farming the Hyde Park is a absolute pain, so, here's your chance.

And if you are not in Camelot/America yet, the only realistic place for Medals and Horseshoes is there.

(to bad there's no gears...)