If you find YELLOW BANDS appear while scanning some printed postcards this is due to a printing method that produces these 'Moiré Patterns' when they are scanned (also happens when scanning pictures from magazines).  To stop this happening try to find a 'Descreen' option in your scanner settings & scan with that activated - this will prevent the pattern appearing & also clear up other 'interference'.  If you do not have a Descreen facility try scanning at twice the dpi that you want, blur the picture slightly, reduce to size you want by resizing & then sharpen - experiment, the results will be good enough for showing your pictures on ebay.  You will find more technical information by putting 'Moiré' into a search engine.