With a core edit I guess that'd be possible.
So what you want to do is go into ItemHandler..
More incoming helping him on Livestream and posting the rest here in 5 minutes for further refferences.
..Guide thingy:
Go into ItemHandler.cpp
Delete this code around line 591-594
Code:
else
_player->SendSellError(SELL_ERR_CANT_SELL_ITEM, creature, itemguid, 0);
return;
and this code at around 560
Code:
if (pProto->SellPrice > 0)
{
and it should work.