I want to know if I would be getting ripped off if I bought the cheaper lens?
No I don't think you will get ripped off at all...It's still being sold via Amazon with the usual Amazon guarantees. What you will most likely get is a lens that was part of a kit. As you know, if you buy a camera and lens together, the price is significantly less. Many stores and individuals will get the kit then sell the lens and camera separately at a large discount and still turn a profit. You may not get the original retail box but you should still have the Canon warranty I believe.
I would not hesitate to get it from the cheaper one.
It's the same lens, just different sellers. The higher one is the MSRP, or close to it. But since they use this lens in a kit it goes for less. I have no idea who Celltime Inc is, but it's fullfilled by Amazon, so I'd feel reasonably safe. Just look into their return policy.
With the recent release of the 6D and somewhat recent 5d3, the market is flooded with these lenses from kits; I wouldn't pay more than 800 something for it. Getting it through Amazon is a bonus over eBay. If the seller checks out I'd go for it.
Edit: DH beat me to it.