Be very careful. Purse.io offers 5% to 40% off amazon purchases, and lots of carders use their service. This means that many have received goods paid with stolen credit cards - and some have got into a trouble because of that.
Up to 15% i consider normal but I have seen also people offering service up to 25% or even 30%.
If i deal I will consider to ship to them first, take some pic and then reship that item to me which I think is safer.