Hmm.. the 50 you can get for 80-90 new, and compared to the Sony one again you loose a metal body, OSS, autofocus and a little bit image quality (the SEL has better bokeh). But of course the Sony one costs 300 bucks vs 80 for the Canon, so you have to decide weher you buy it or not. Get the used one if you want to help your friend and if its in mint condition, otherwise buy a new one (but then again, in this case a legacy lens would almost certainly be the better option).
With the 75-300 it's a different story. Again, the price isn't a bargain for a used one, and the lacking OSS is a huge problem at this focal length an aperture speed. If you buy this, consider that you'll just be able to use it without a tripod in the brightest sunlight.