pick up, gather
摭 centers on the action of picking things up and gathering them together, often by hand.
摭 combines 扌 (hand) with 庶, which likely contributes the sound and may relate to spreading or arranging, suggesting gathering by hand.
The hand 扌 beside 庶 (which can suggest many or common) gives the image of a hand picking up many scattered things and gathering them together.
For セキ, imagine picking up a stack of papers and saying 'check' as you gather them: check -> セキ, and your hand collects the pile.