Should I sell the seal of hallownest?
The only thing you can do with Hallownest Seal, Wanderer's Journal, King's Idol, and Arcane Egg is sell them to Relic Seeker Lemm for Geo.
If you don't need the Geo right away, consider waiting to sell them because unlike your Geo, these items are retained on death. You can think of them as a safer way to keep large amounts of Geo.