Explain to your customers that trade-in offers are based on the supply and demand observed in the second-hand market.
However, the selling customer benefits from using your service compared to other solutions on the market: guaranteed voucher within 15 days, available customer service, no negotiation or exchanges with buyers, no product sheet to create, free shipping label.
It is normal for your brand to include the costs associated with all the efforts the customer did not have to make to sell their item: product sheet, packshot, dry cleaning, repair.