Set completion tracker is at /sets. Pick any set you're collecting — the page shows every card with a checkmark if you own it (any grade), greyed out if you don't.
Click any missing card to open its detail page → Find on eBay button searches eBay for that specific (set, card number) combination. The Sniper will also start surfacing deals on missing cards from any set you mark as actively collecting.
The "% complete" badge at the top of each set page reflects unique cards owned, not quantity (owning 3x Charizard counts as 1 toward completion).