(For clarification, I did not model this map it is the Pet Simulator 2 map I believe, and before I get responses about using others work it is just for example purposes as I am not selling this graphic.)
I do enjoy the low-poly art style so my apologies if this comes off a bit bias, but I believe this work is impressive and extremely well done. The soft glow of the sun really adds a bunch to the whole picture, please keep creating!
I am in love with the lighting and the low poly style, I think that you chose a very good setting to make the graphic design. It is pretty impressive considering this is your first ever Graphic Design, if I were you, then yes, I would consider on getting into the graphics designing field in roblox and selling your creations.
On the price side, I would say that if you were to sell this graphic design, I would say that you could go for something in the area of 400 to 1000.
Yes, you should get into the graphic designing field of roblox, and before you do that, maybe you could make around 2-3 more graphic designs and ask for feedback on them in devforums, and if people will like it as much as this one then you should get into the graphic designing field. (Only if you think you are ready)