At that price, especially in Bolivia, it'd be easiest if you went to any bike repair place and said you're thinking of selling it. It'll be gone the first time you say that. They don't care about paperwork at all.
If you want to sell it to another traveller it'll be a CARTA PODER. You need to go together with the buyer to a notary and get a carta poder por conducir en todo países en sur América. Then get that notorized. It's easy, not even a full day if you can find a decent notary (some will say they can't do it, just persist).
Its not possible to transfer the actual ownership without you both being in Colombia. But people have travelled for years on a poder so don't stress it. When the buyer is done with the bike, he can either try get it in his name if he's in Colombia and sell it on, sell it on a double poder; or, for that money, probably sell it to a farmer.
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