In case anyone is confused, the $500 ARIN fee is per transaction, not per IP.
$48 x 1024 (IPv4 /22) = $49,152. For that transaction the ARIN fee is effectively 1%, considerably lower than the commissions charged by the brokers (which also comes out of the seller's proceeds).
The ARIN fee covers their staff time to review the transfer - specifically the history of the legal entities involved, which as anyone who has dealt with ARIN knows is extremely thorough.
ARIN does not frown on this marketplace, in fact they encourage it and even endorse specific brokers.