Both sides, priced the same way
Every card in the app carries a market value from the same daily catalog prices. That goes for your cards and theirs. As you add cards to an offer, TraderPost totals each side and shows the gap in dollars.
A trade reads as fair when the gap is within 10% of the larger side. Past that, the app says roughly how much to add.
Market value is a reference price, not an appraisal. Condition, grading, and what you actually want out of the trade are yours to weigh.