Chances are your lender is not going to allow you to walk away from the closing table with the keys to a home and $15k in your pocket (they have gotten burned where Buyers were going to use the money to fix something and instead spent it on a vacation and the lender ended up with the home through a foreclosure proceeding without the repairs being completed). A good faith estimate from a recommended lender will give you what the estimated closing costs and pre-paids will be - from this document, you then are in a better position to determine how much of your closing costs you need or want the Seller to cover. The Seller will look at your request, deduct anything you have asked for from the offer price, and then determine if the net offer is attractive enough to accept, reject, or counter.
Good luck with the pursuit of your first home!
The $15k will probably be taken from the proceeds of the sale of their home so you won't see a dime of it. If you're making an offer, you can simply offer $15k less than asking price if that is what you wish to do but you won't be getting any closing cost assistance.