It’s not a fold – the top tube on the sb66 has an intentional bend in it midway along (roughly at the point where the bars would touch when turned 90°). To me, it looks like it’s all happening in the same place which makes for a tricky dent to photograph accurately.

Absolutely no way whatsoever at all would I entertain buying a bike with that damage. It’s no longer a bike.
Why not send the pic to Yeti and get their opinion?

There are other pictures of the dent though and the others give a different impression, do you see what I did there :0)
Didn’t like the wording of the ad but the price can’t be far off. The rear end and shock could fetch a fair chunk of that.
Doesn’t look like serious damage in this picture.

I don’t know why people commenting on the dent haven’t taken the time to look at the pics.
They are here: http://www.pinkbike.com/buysell/1473677/

These two in particular make it look reasonably trivial:

There is one that makes it look fairly bad though – in that you can read a lot into it:
This one makes it look particularly wrong:

A couple of years ago I sold a dented 09 Spesh Enduro frame for £170. I happily rode it but boy I wouldnt pay/sell it for anywhere near its ‘market’ value. All I asked was the buyer didnt strip/fill/respray and sell on as mint.

Fine if you damage your own bike but price is accordingly. Nice frames them Yeti’s but more fashion than the best ones I bet.