I needed funding to continue with my PhD studies, so applied for it. Was hired because senior graduate student and had required qualification for the course. Plus funding constraints. No negotiations are required. Its a fixed salary, same for everybody. The experience is pretty good, plus you learn a lot about the teaching methodology and course material. Its worth doing atleast one graduate teaching assistant in your student carrier. Undergrad TA can also be useful.

Based on GPA and studies in undergraduate program. They might consider the TOEFL/IELTS and GRE score at the start. Depends on the the hiring team, also the prefessors recommendations might help. But cannot assure.