In my searches I have come across two different Xboxes at pawnshops but when I ask to see them to check the manufacture date I see that they have been opened. For $90 would you take the chance? Since they had been opened, I assumed that the previous owner tried modding it because what's the point of opening it up for any other reason. If it boots up to the microsoft dash the worst I can think of is that the screw had been stripped. Would you purchase the open Xbox or keep looking for ones that were not? I suppose if there is good indication that the DVD-Rom drive is a Samsung then I would buy it.
Plug in the power and see if it boots up to MS Dash; Open the DVD drive tray and see if you find one big and two small holes on the tray. If everything as described, buy it. It's a good deal at $90.
I would replace it with one of these: http://search.ebay.com/Modified-Samsung_W0QQfkrZ1QQfromZR8 EDIT (By Pop_Smith) If i was keeping the Xbox, i would get one of those, as most used XBox's DVD drives are sucky.
make the pawn shop prove they are in working order. the only problem I can think of is it might have a dirty disk error or something. also if you are planning on playing LIVE at all i wouldnt go this route. people get banned and sell their box.
I would take a chance, but I would make sure everything is in good condition. Not so much appearance, rather the xbox powering up to the ms dash and the dvd rom working.. You can always replace the drive but if you pay 90 bucks and the drive is 70 or 80 bucks, you could buy a Brand new Xbox for that.