The knives I have just come from a second hand shop, and are quite blunt.   Is it possible to sharpen them using an ordinary sharpening stone/steel, or is there a point that they are 'too far gone'?  
Very blunt knives need to be reworked by stone or sandpaper The edge on a very blunt blade will have folded over itself and lost a lot of its proper crystal structure.  It's also likely to have lost a lot of its bevel geometry too, so the edge won't be "straight" longitudinally. Your best bet would be to invest, just one time, in sending it in for professional sharpening, then keeping it sharp using leather stropping or a honing rod. Don't try to sandpaper it as there is no way to ensure proper bevel geometry with sandpaper without some professional equipment. And...sending the knife in will be cheaper than buying a set of wetstones (which most folks don't know how to use properly anyway). Here are some images of folded and blunt edges:   