It seems that Neymar has been removed from the market following his move to Santos. Since the Brazilian league isn't featured in the game, his cards have become untradeable—similar to what happens when players retire and were not good enough to be a "hero" or an "Icon". This has also occurred with high-profile Prime Icon cards like Pep Guardiola where people lost a lot of coin. Occasionally, valuable cards may become untradeable for this reason. You’ll need EA’s final confirmation, but in cases like this, when a player transfers to a club that isn’t in the game, their card often becomes untradeable. This does mean your coins are gone.
Look at it from the perspective that you do have a relatively high OVR Neymar card that most will not get a chance to get, likely ever (until he does get the Icon card post retirement, moves to a club that is in the game or Santos gets added to the game in might get a Libertadores card or maybe his international Copa America / World Cup type card..