No problem to enter Nepal with Indian bike, but you should register with Nepal customs on entry as you have to pay Nepal rd tax. You can sneak the bike in, but this could create problems when you leave, unless, of course, you sell the bike in Nepal, in which case the buyer will inherit the problem !
I.ve bought and sold several Indian bikes and never had one in my name, possesion of the papers is considered proof of ownership and most foreigners tend to pass the bikes around between themselves. However a properly filled in and notarized vehicle transfer form from the last registered owner would be required if selling to a local (Indian), and should, therefore be required on purchase.
For selling the bike in Kat, try www.hambrobazaar.com
, it's kind of a Nepali ebay, lots of vehicles. Again, you would really be looking to sell to a foreigner, as the bike is Indian reg, for a Nepali to buy , he would have to import it and pay the 300percent import duty, so you really want a buyer who will take the bike back to India.
Finally, NOC, No Objection Certificate, means what it says, can be issued by many different departments for many different reasons, in Nepal, for example, if you wish to purchase a bike as a foreigner, you require NOC issued by your embassy, before you can register it in your name (yes, in Nepal this is possible),in Manali, if you want to cross the Rothang pass on a non HP reg vehicle you need an NOC from the local sub magistrate etc.