As others have said, you do have Buyer Protection from Ebay and, if you paid by PayPal, protection there too. It certainly looks like a 'hack' job but not one that can't be fixed by a professional - assuming you also would want the marks around the nut sorted too.
In the first instance, you should contact the seller, send them the Pictures and tell them you are not happy at all. Maybe they will refund some of the money back - assuming you wish to keep it - enough money to get the problem fixed by a professional. If the marks don't bother you and you are confident to replace a nut yourself, you could do that too.
Point is, and as others have said, there are course of action you can take. If you are not at all happy and don't wish to keep the guitar at all, then you can return it for all your money back (less any shipping costs you incur). You can contact the seller and see what action they are willing to do - offer a partial refund for example. If they don't, then you can look to return and get a full refund. I know both of these are additional hassle you shouldn't expect but at least there is 'some' action you can take. Its not like buying an instrument via some 'classified' ads (like the old small ads in newspapers and similar type websites like Gumtree or Craigslist). Buying from Ebay does offer some buyer protection but the first course of action should be to contact the seller, show them the Photo's and either demand a Partial refund or at least ask them what they are going to do to rectify the problem. If they refuse, then you can get Ebay (and/or Paypal) involved.
Whilst Ebay can provide a great way to get used instruments cheaply, there is always a risk that you can also get someone elses 'rubbish', something that isn't quite as you expected or hoped it would be. Ebay does offer some buyer protection but it can have people selling their 'trash' like an online yard sale. Buying used from a 'store' is generally the safer option as they tend to offer some warranty, possibly even checked that the items are fully working and fit for use and easier to contact in the event of an issue. Buying online from a store often has better protection than buying in store. In the UK at least, there is an option to return almost anything within a short time (14days I think) and get your money back - even if the item is 'perfect' - of course it has to be returned in the same condition unless it 'breaks' through normal usage.
Personally, I would never buy any Used items, especially not electronics like TV's etc - from any 'individual' seller and certainly not without seeing it and/or getting to use it first hand. I would be very wary of buying used from a store on a lot of items although less so when it comes to high end Electric Guitars. I would never buy any Electric guitar new or used under £1k without inspecting it and playing it first. Generally, Stores are much more inclined to inspect, check and ensure everything is 'perfect' on higher end instruments - including the electronics. You never know if 'everything' is stock and has always been stock too with used Instruments. Just because it has the Original PU's in it, doesn't mean they weren't put back in just to sell and you don't know how competent they were or what else they may have messed with too.
I still prefer to buy New - if I can't afford the guitar today, then I will keep putting more money aside until I can afford to buy. I prefer to buy PRS new because of their Customer Support and after care - should anything go wrong. I may never need to contact PRS over my guitars BUT I have piece of mind because that option is there if needed. There is also the fact that I am supporting PRS, not that I matter in that regard as PRS will no doubt be equally as well supported regardless, but also know that should anything go wrong, PRS will support me in getting that issue sorted, like a member of the family. That to me is worth the 'extra' to buy new. I know not everyone will share that sentiment, prefer to spend less and get a great used deal. I am not criticising them or saying they are wrong to do that, just saying that I myself prefer to buy 'new' if/when I can. Saying that though, I wouldn't mind adding a 513 to my collection - just because.. but would have to buy used now. No rush, I am waiting and saving for a Fire Burst Red 594 Hollowbody to come to the UK...