Welcome to Gaia! ::

I think it depends on what else you have to sell... if you don't mind filling up your store slots with fish, go ahead.

Personally, I've long since given up on selling small fish in the marketplace and go to Logan with them.

you could always sell your pebbos and blu seedkin to me lol razz i'm almost always buying pebbos (12g each) and blue seedkin (10g each)

I sell fish on the marketplace all the time and it varys from item to item. Here are a few tips on selling on the Market that you should use.

If you play your cards right in selling on the Market you can easily pay for your bait and get a profit selling the small fish alone. Higher priced items will take longer to sell but as long as you have a fair price on the item it will sell in 1 to 3 days. Also, higher quanitys of an item sell for a bit more and are more sought after. If you want to sell off a Helmet my advice is to sell off 100 of the required fish instead of turning the fish into an item. It sells faster that way for some reason.
There are a few mistakes I made when I first started selling fish that I would like to list. Selling fish to logan is horribly in effective. You also don't want to accept offers from people looking to buy your fish for what Logan will give you. A prime example of this is people looking to buy your Guppies for 3 gold each. Turning your small fish into medium will not yield any profit at all. If you want to get your hands on medium fish the best way is to learn how to filter there are some very good guides on how to do this in the fishing forums. I recommend you stick to mostly F-Class bait until you get the hang of fishing.