The Game incorporates a lot of rather complex mechanics, like for example Amity for an MMORPG, and it is kinda similar even with selling fish. Unlike Trash-loot, fish are being sold to one of to types of NPCs, that I will tell you about in a moment.
So where to Sell Fish in BDO?
Fish can be either sold to a Trader Manager or an Imperial Trader, both of them accept fish. The Imperial Trader does not take any fish lower than T2 (blue), but I would very much not recommend to waste your time and only sell fish that at least have a yellow grade (t3). Our Black Desert Online Bot is automatically sorting all picked up fish, to not unnecessarily waste any inventory space and fill them all with Ancient Relic Crystal Shards and yellow fish
Trade Managers can be found in any town without an exception. The price they pay for fish ranges in between 80-130%, so depending of the size (and amount of fish) of and in your inventory, it might make sense to look for the best paying one – but in most cases my recommendation is to just sell them as fast as possible. Like with other tradeable items, you can bargain with the Trade Manager to get some additional % in profit, which you can do in case you have some spare energy top waste.
Imperial Fish Traders
Unlike Trade Managers, Fish Traders are rather rare. They additionally only accept blue and yellow fish, which should be no problem since all fish grades below really pay so little, that you are better off just throwing them away.
The awesome thing about the Imperial Fish Traders is that they give you a whopping 250% of the Fishes value, which, in some cases, triples the amount of silver they pay you compared to normal Trade Managers.
It is not worth it to travel half across the continent only to meet them, but if you are close, it is definitely recommendable that you sell your fish to them.
In total, 5 Imperial Traders exist, namely in Velia, Valencia City, Splashing Point, Epheria Port and Glish.
How to Sell Fish in BDO
Lets say you threw away all your white fish, and are only left with some sweet blue and yellow fish.
Go to either the Imperial Fish Trader or normal Trade Manager, talk to them pressing R, and you will already see the option available to sell your fish.
Bringing a Fish all the way from Calpheon to Valencia would increase your gain by 60%, which is barely a million silver, so my advise is to just sell your fish always at the closest trader.