I think its the trader. I've seen some applications where you had pay on another site to unlock a code for a application.
I would always advise checking reviews for said before any purchases or just not to buy that application if you are unsure.
OnePlus 5 review
Alice Saey's mesmerising animation for Dutch singer Mark Lotterman
iPad Pro 10.5in (2017) review
Comment booster votre iPhone ?