Sadly, there is nothing cool you can do with the trophies you collect in The Witcher 3. Geralt has no storage area for them and once you finish all the contracts then they will be sat in your inventory taking up space. The only solution is to take them to a vendor who will buy them off you for full price.
An alchemist inside a church located near the Oxenfurt Gate shown below will buy your trophies off you for full price. He has only got 500 crowns and takes 10 days of meditation to restock his gold supplies so you can either sell them then meditate for a while or keep coming back during your playthrough (or buy some alchemy goods off him, as he has some quite useful items).
The alchemist only appears between 5 and 6 AM. Sometimes he will bug out and not speak to you. If this happens then leave the area, meditate a while (a day or so worked for me) then come back. Some people have also said you can meditate next to him but I wasn’t able to make this work.
This vendor will not only buy trophies for full price, but everything else you might want to sell so you will want to go use him a lot if you want to make the most money from selling your loot. Check out the video below if you want to see his exact location.
I hope this helps you find where to sell all your excess trophies. I know this is not the only location you can find to sell items for full price but it is a very convenient one with a fast travel point very close. The only downside as mentioned is his amount of gold, I hope that new vendors are added into the game to make selling expensive items and equipment easier.