Why people look elsewhere
PrizePicks gives 24 hours from opening a pack to sell back or ship, after which the card sells back automatically at 100% FMV. If you'd rather think for longer, or don't use PrizePicks already, the apps below work the same way with different clocks.
Underdog Rips
Underdog sells packs from $5 to $2,500 (Pokémon to $5,000), each tied to one vaulted card, and pays 100% of the FMV shown at reveal. You get 3 days to decide, except on $1 packs, which sell back after 60 minutes.
Rips by Triumph
The App Store listing says Rips sells PSA-graded TCG cards and buys them back at 100% of fair market value. Sale proceeds can be withdrawn after identity checks, with instant withdrawals averaging about ten minutes, per Triumph's docs.
Packz and Packed
Packz buys revealed cards back based on FMV and, under its current rules, gives 72 hours before an automatic sell-back; proceeds can go to your bank. Packed says it sells back at 100% of market value and runs live rips with hosts, but its Vault timer is short and deposits can't be withdrawn.