1. Depending on the phone and root method will determine if you can unroot and restore to factory defaults. In 90% of the cases you can, although before you start flashing make backups and be sure to do research on rooting, as well as unrooting and be sure to grab any unrooting files just in case the links go down (had this happen on my Droid X) Newer phones come with an unlockable/lockable bootload which will play a part as well (unlock bootloader, root phone, unroot phone, lock bootloader)
2. The value wont decrease, only your warrenty will go away. If you're selling the phone rooted, to certain crowds this will increase the value, to other crowds it wont change the value because they don't know what it is. But in any case, doesn't hurt to unroot and then sell as well.
What kind of Android do you have? I can link you to a reputable site to get the files you need for root/unroot if it can.
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