The first question to ask yourself is what you think a FF camera will do for you, that your current crop camera won't or can't do? Full frame would be better in low light (i.e., less noise at high ISOs). And FF is great for big prints... from about 16x20 and upwards you'll see some differences. If all you ever do is make 8x10s and share your images online at Internet resolutions, the FF camera would be sort of a waste.
A crop camera such as your 50D has some advantages, too. If you do a lot of telephoto work, for example, the cropper can be a better choice. You can use lighter, smaller, more portable, handholdable and affordable telephotos with a crop camera, than you'd need to achieve the magnified angle of view with any full frame camera.
6D's AF system is similar to your 50D's, in some respects better, in other respects not as good. Your 50D has 9-point AF, with all nine the "better" cross-type (dual cross at the center point, with f2.8 and faster lenses). The 6D has 11-point AF, but only the center one is cross type. All the others are less sensitive than your 50D's peripheral points. The 6D's center point can still focus in exceptionally low light... down to about -2 or -3 EV, where your 50D is only able to focus down to -0.5 or -1 EV.
6D is sort of like a "full frame 60D"... with similar controls to 60D and, to a little lesser degree, 70D. Instead of the control dial on the back of your 50D, these cameras use a multi-controller. Some folks prefer the 50D style setup, for quicker action in fast shooting situations. For action and sports, your 50D also can shoot at over 6 frames per second, while 6D is only able to do about 4.5 fps.
6D uses SD memory and LP-E6 batteries. So you would need new memory and any spare BP511As you have now, plus your current charger, won't work. You would get some more shots per charge... I estimate I get about 20% or 30% more shots per charge with my cameras using LP-E6, compared to earlier models I used that utilized BP511A batteries. LP-E6 are more expensive, too, especially since cheap clones of BP511A are widely available and work just fine. (I only use OEM LP-E6, but have seen some people have issues with "fakes" or clones. Apparently LP-E6 contain some "smart" circuitry, so aren't as easily copied as "dumb" BP511As.)
Maybe a 6D is perfect for you... Maybe not. Depends upon what you want to shoot with it and how you use your images. Only you can say
Like some other responders... I would be very, very, VERY suspicious of such a low price on 6D. It's normally a $1500+ camera... hard to imagine a brand new one for $1000.
Plus, depending upon what lenses you already have, there may be significant additional cost to "going full frame"... On the whole, lenses for FF cost more and are bigger/heavier than some of their crop-specific lenses alternatives.
EDIT: You don't mention the seller.... but if they are online you might try looking them up on
www.resellerratings.com to see if there is feedback about them. I get even more suspicious when I hear about a seller who is "bundling" a bunch of stuff with a camera. Often it's unnecessary or misrepresented cheap stuff that really doesn't add much value... such as small, wobbly tripods or poor optical quality add-on macro/wide angle/telephoto lenses.
The Canon USA website lists exactly what's included in the box, with any particular camera. If the seller is "bundling" a battery, charger, strap, USB cable, those are usually included anyway.
A non-USA (i.e. "gray market" ) camera will not have warranty coverage in the US. It was bought overseas in another marketplace, then imported through non-official channels. If it breaks, you're liable to be stuck with the cost of repairs. The "one year" warranty is between you and the seller... not between you and Canon. Some of the long-established, reputable sellers offer gray market items and provide their own warranty and are good about standing by it. Not knowing who you are talking with, hard to say if this is a good deal or not, or if they'll be around to cover any warranty claims, should you need them.
There occasionally are screamingly good deals on things.... But most of the time if the price of a camera is significantly below what the largest retailers (B&H Photo, Adorama, and a couple others) are charging, it's just too good to be true and is a rip off in one way or another. A USA-warranted 6D at B&H is $1700 right now (after a $200 mail-in rebate). They (the largest camera retailer in the US) can't get gray market cheap enough to offer them as "body only", so they offer in "bundles" such as with battery grip, extra battery, memory card, etc., with a 2-year "Square Trade" (third party) warranty, for a few $ more than the above price.
So... search online by the seller's name... and at the resellerratings.com website, Yelp, Better Business Bureau, etc. for any info about them. You also might research the seller's location and, if it's in the U.S., check with local D.A. office to see if there are any consumer complaints.
$1000 for a camera everyone else is selling for $1700? Sounds too good to be true... and probably is. But, who knows!