Selling a bike should be no different than selling a car. Many people drive cars down from Europe to sell them in West Africa. The most popular places to do so are Mauritania, Mali, and the Gambia or so we have heard. Guinea Bissau might also be an option.
Mauritania is the most popular because it's closest to Europe. They stamp the vehicle in your passport but with a bribe you can prevent this or get it taken care of after you sell the bike. We hear that it has become more popular so you might not get as much for your bike. Then again, not many people are selling bikes, mainly cars, vans, and trucks.
Mali is supposedly easier because there is no carnet required and they don't stamp the vehicle. It is farther however and you might have to go all the way to Bamako to get a good deal on your bike.
We also heard the Gambia is one of the best places because there are a few people who will take care of the whole process for you. There was a post on the forum a few weeks ago about selling cars in these countries, do a search and you can get that information as well.
Morocco would be very difficult because they stamp it in your passport and cops are less corrupt. On top of that, the system is computerized and they will find out as you leave the country. Import costs are very high so you might get in a lot of trouble.
Senegal is also complicated unless your transalp is newer than 5 years. We have carnets so they filled them out and we didn't see the procedure for non carnet users. I think they just fill out a laissez passer and you can enter if the vehicle is less than 5 years old or if you cross at a border other than Rosso or Diama. Once you get in without a stamp in your passport, selling it should be pretty easy. Go to the port in Dakar, a lot of vehicles are sold there and crooked customs guys are hanging out there as well. You'll need to bribe one to take care of the paperwork in case your buyer won't do that for you.
Lastly, you might want to look into Guinea Bissau. I am currently in Bissau and it was the easiest border crossing I have ever been through. They didn't even look at any paperwork, just waved me through. I am in the country now with no paperwork saying I have a bike, so selling it would be easy from that standpoint. However, there might not be much of a market here especially given the current gas shortage and poverty.
I should say that I haven't tested any of this, this is just what I have heard from forums and travelers we have met along the way.
Good luck with your trip and let us know what happens.