Why isn't my mini fridge getting cold enough?

I would give it another day or two to 'settle down' and run in a normal mode. It may just need a few more hours to get the refrigerant circulating good. If, after that length of time, it still doesn't get any cooler than 50, and the coils are clean, I'd have to agree with you and say you got ripped. Could have been an older unit near the end of its useful life that someone was looking to unload for some quick holiday cash. Welcome to craigslist.

Does the seller offer a refund? Is the unit under warranty?

Minifridges are not a good deal even when they are brand new and operating normally because they waste SO MUCH energy. It's a joke how much energy they hog up compared to how little they can possibly do.

Okay, here's what is probably wrong with your little mini fridge:

There is probably not enough refrigerant in the coils. Have you ever had a car that had its air conditioner stop working and they either found a leak or said it was just "time" for a recharge? They literally have to add some of the chemical that circulates through the guts of your fridge, getting compressed and expanded and compressed and expanded.

Does the little fridge keep making noise, like kicking on and running it's little motor constantly? It is trying so hard! It is trying!

If it is not "running" a lot then...

The other problem could be that whatever device the fridge is using to detect its temperature could be broken, or stuck, or gummed up such that it THINKS it's cold enough and kicks back and relaxes when it should be working to get your stuff cold.

You should be able to tell which of these is your problem by whether it keeps working hard all day long, or seems to be just relaxing.

The first case would mean you are wasting a LOT of electricity. The second case means you're not wasting too much energy, but the fridge is just a big paper weight until you fix it.

Edit: Mike is right: having jugs filled in your fridge/freezer would help stabilize its temperature, but in all likelihood you don't have too much available space in your tiny little minifridge. If you have ANYTHING in there it should be enough to stabilize it.

For the record, better than jugs filled with water would be jugs filled with any kind of electrolytes: salt water would be great. Sugar water (yes I know--not technically an electrolyte) would be fine. Just dissolve as much salt or sugar as you can into your jug of water! Or if you have a big jug of Gatorade that would be alright.

Gatorade Mini Fridge

If it doesn't have coils in the back, it is not going to get any colder. The really small fridge's like Haier and Chef mate do not get much colder than about 45 degrees. It is because they are not made like a regular fridge. I wanted one, and it was given to me. I'm glad I didn't buy it or I would have been disappointed. If I get another one, I'll get one of the slightly larger ones that has coils like a regular size fridge.

Is it empty? It may need something inside it to hold the cold in. Like my fridge, it used to cycle on and off all the time. Being single I did not have much of anything in freezer. A friend told me to save plastic gallon milk jugs, rinse out, fill up with water and place them in freezer. Now it cycles about half what it used to.