Well it is runescape, so most likely legal unless it involves interferance with another person somehow like DOS. Also you sure are worried about a lot of things being legal or illegal.
This guy refuses to show me his bank because he doesn't want to "show off"(He is poor and probably hasn't done the guide himself or it doesn't work) Then later he says he won't because he is 9 hours from 99 str, so he won't take 10 minutes of his botting. Don't buy this guide probably doesn't work, or its very undoable like the buying onyx ammys and enchanting. Or he is just scamming