Don't be discouraged so easily! :c
I probably wouldn't be paying upwards of 75TBT, I'm sorry to be so blunt but that's just my personal opinion. Hear me out though, because it could be worth a lot more if you take some criticism and follow through.
It's clear you've got the hang of colouring and basic anatomy, that much is good, but it's just a case of refining. I'm by no means an art expert (I'm only 14 for crying out loud xD). I like the shading on the shirt, but the shirt isn't the only thing that has shadows! Think of your light source-where is it? It's from the right then you'll need more shading on the left.
Also, think of where you can imply creases. Creases aren't just straight lines, they're all ziggy-zaggy and rumply! Add some folds in the shirt and jeans. As well as this, the face has some crease-like contours. Under the eyes (cheeks), cheekbones, chin, eyelids.
The hair is pretty good though. You've got the texture spot on-it just needs a bit of extra volume and flow and we're there.
Also the anatomy is a bit off. The torso is fine but the legs need a bit of work. Your legs don't go straight out when you bend them, they
bend at the knee and your feet bend at the ankles!
The only other thing is the digits. To me it looks like the hands have four digits. Most humans have five! (Unless you're making an four-fingered OC, that's totally fine c

The shoes are a bit stubby-the ankles seem to go straight down and there's no heel. There's a curve from the bottom of the ankle to the foot-in the drawing there's just a little bump to indicate a foot.
I know this is most likely a chibi, but even chibis need their own more-human-esque touches!
Hope this helped, and good luck! c:
Mariah's known for being blunt. Not rude or disrespectful per se, but there's no messing with them :L