To whom should I return my trophies

In Witcher 3, the trophies description says that it can be returned for a reward but I am not sure who to return those trophies to.

Note: This vendor was patched in version 1.12 (released in January 2016). The alchemist no longer buys at 100% of cost. Leaving the answer up as it might be relevant to GoG purchasers who haven't patched.

The alchemist near Oxenfurt Gate (north east of Novigrad) will buy trophies from you, for a better price. He also sells extremely reasonably priced precious stone dust (and other alchemical ingredients).

He turns up at around 5am (you'll need to be outside of the building, otherwise he won't spawn).

He'll actually buy anything for it's full price, though he has a limited amount of Crowns.

After killing the beast you usually return to the one that gave you the mission to collect your reward but you don't hand back the trophy to him/her, you just keep it yourself. You can then sell it or equip to buff some stat.