If you are familiar enough with bikes in general and can inspect it to determine if the quality of is sufficient and the bike is in good repair, your only other consideration is fit. If it fits the both of you pretty well, $500 is a bargain. This sounds like a good starting point to see if both of you (especially the stoker) likes tandeming. If he is willing to let you try it out, you will probably be able to see if you like tandeming in 10 or 20 miles. You will also be able to determine the quality of the bike. Good luck!