Hi biggysmalls35. I can only comment on MJConcentrates carts as I do not trust the American ones at all.
You want a vape battery that uses a '510' thread and that you can control the voltage with as thc carts work best between 3-4volts anything over this and you run the risk of ruining the cart. Amazon sell a vape pen battery called a 'honeystick 510 twist' it's about £10 and has a base that twists for desired voltage. I'm sure there's others available but it's the 510 thread and the ability to control voltage which are the really important points with the battery.
With being ripped off or not, check the ratings and reviews for vendors and the carts they sell. Like I said I've only used MJc carts and the rating and feedback speak for themselves with that product. American dispensery carts are counterfeited all the time with identical packaging easily available on the net. Without lab testing you really do not know what your vaping (or smoking or eating for that matter we all use lb with that risk) but I've used MJc's carts for about 5 months and experienced no adverse effects.
As a heavy user I find 0.5ml cart lasts about 3 days, a 1ml will last about a week. I find the onset for carts to take a few minutes but the high is clean and strong, not as long lasting as a joint so bear that in mind. I mostly use these when I have no flower, if I'm out and about and before bedtime as they get me super sleepy.
Sorry for long post, wanted to be thorough, hope this helps.