I dont think so, jpg is a lossy format, meaning that anything converted to it gets a quality loss in favor of less size; thats a big no-no for game art, specially 2D. Just stick to PNG, people will shy away from most jpg and it wont reflect the real work. Renaming *anything else* to Jpg is a dumb move, the only exception i can see for the use of jpg is for humongeus textures where the quality loss doesnt matter much.

Can i make a crazy suggestion for next challenge?

Go in here:

http://orteil.dashnet.org/gamegen

And make a piece of art about whatever you are first told. You get a second change if you get something too nasty or boring.

i think you should avoid too much keyboard input as a rule, and limit the haggling to a up/down button that will apply a % depending on your skills. That would help to keep things dinamic, since the first 4 times it migth be fun, but at the hundreth you just dont care anymore and buy/sell at base price save exceptions (old Reccetear player here).

Also giving a backstory with a few endings, i played not too long ago a funny game about a vampire  turned shopkeeper by the hand of a belmont kicking him out of his castle. With your trading you decided which realm wins an ongoing war by selling weapons and armor or giving loans to a side (or just sell to both and rake in the money). You can borrow that and a few of your own as an endgame goal instead of the common huge debt goal.

Your drawings are not bad, to my untrained eye they just seem in need of a bit of polish; sadly this is not the page for you to learn more, you will have to head to some other forum.