I've trolled other forums about this question and everyone seems to find it weird. Here's how to approach it anyhow:

Fixed component = 1000
If x > n, Variable component = C (x - n) Where x is the number of sets she sells
Total income = 1000 + C(x - n)

If x < n, Variable component = 0


1. If x --> x - 3, Total income would have been 600$ less

i.e. 1000 + C(x - 3 -n) = 1000 + C (x - n) - 600
=> 3*C = 600
=> C = 200
Can we find "x" ? No. INSUFF

2. If x = 8, Total income > 4000$

i.e. 1000 + C(8 - n) > 4000
=> C(8 - n) > 3000
That doesn't really help us much does it? --> INSUFF


Both 1 and 2:

We have : 200 ( 8 - n) > 3000
=> 8 - n > 15
=> n < -7 , which is absurd, leading me to believe this question might be inherently flawed.

However, being as it is, I'd go with E.

albatross86 wrote: Both 1 and 2:

We have : 200 ( 8 - n) > 3000
=> 8 - n > 15
=> n < -7 , which is absurd, leading me to believe this question might be inherently flawed.

Statements (1) and (2) contradict each other, which make this an impossible question (i.e. the answer isn't "E", it's "this question would never show up on the GMAT because it's flawed).

What's the source? You should always provide the source when you post a question.