There are several ways to go in tying flies that you want to receive some kind of payment for.
First, as Narco dog pointed out, there are laws involved with selling things for profit, some are specific to fishing related products, and others to your state and county regardless of the product. WE all know about the IRS, too.
Be sure to investigate and follow these rules. Doing so makes sure you both stay out of trouble and keeps the playing field level for all (if the other guy you may be competing with IS adhering to all the laws, and you aren't, you are cheating him...don't be surprised if he turns you in).
And, yes, it doesn't matter how much you make, or how many flies you sell. You should still follow the rules.
Selling the flies in bulk for cash to a shop or catalog is one way to go. Hard to do unless you have some unique flies or are very fast. Easiest way I know to do this is to specialize in locally preferred flies for specific species or waters.
Trading flies to a shop for credit in the store. Lots of folks do this. It works well for the shop owner, and you can get stuff you want or need. Remember that you are still selling them, and need to adhere to the appropriate regulations. This is probably the easiest way to get some value for your flies.
Selling direct to fishermen. You can build a surprising amount of business if you don't mind building it slowly through word of mouth. This supposes that your flies are well made and work in the waters your customers fish. Many tyers back into a small trade this way.
Remember that while there are requirements to running a business like this, there are also some benefits. Legitimate businesses can purchase supplies at reduced prices. Wholesale is just the upper end of that. These savings are always well below the retail price on things, and if you purchase in sufficient quantities, you an save 60 to 70 percent on many items. Obviously, doing this either ups your profits and/or reduces your prices.
It's still a hard business to make profitable. Lots of overseas competition on what you'd call the standard patterns. Lots of big businesses with incredible buying power that you won't be able to compete with as far as pricing goes.
Some of the hardest working people I know are professional fly tyers. If you really love it, though, you can make it work.
Good Luck!
Buddy
It Just Doesn't Matter....