I cannot tell you about Chile, but to sell goods which are not yours (more preciselly, sell goods one someone's else behalf), you should use a Power of Attorney signed before a notary. I guess the owners already signed a POA authorizing you to "ride" the bike in Chile and abroad (btw, ask for The Hague Apostille to be stamped on the POA so that it is valid internationally). Well, do the same, ask the notary for a "PODER PARA VENDER VEHICULO" (POA to sell a vehicle) and stamp it with the Apostille as well so you can use it in any country part of the Apostille Convention.
In both cases, ask for the "cláusula de substitución", which means that the "faculties/powers" you were given by the owners through these POA, you may transfer them to someone else ("subapoderamientos"), so that the 3rd person (buyer) may be also authorised by you (actually, by the current owners through you) to "ride" the bike and to sell it to a "4th" person (if you do not actually sell it to the buyer, but authorise him to ride it and later sell it on behalf of the US couple still owners to this 4th person).
I hope all of this is understandable. I copy an example of the "substitution clause", adding as well an authorisation to ask for new copies of the POA itself in case you lose it: "Sustituir este poder en todo o en parte y revocar las sustituciones conferidas, otorgar subapoderamientos; pedir y retirar copias de esta escritura".
BTW, the current owners may sign all these POA even if you are not in Chile (well, it's a POA): for instance, they could sign it, stamp it with the Hague Apostille and send it to you (translated) to the US and you could sell it in the US to someone else. So, they should give you a copy of the POAs or you could get it directly from the notary with the clause I mentionned. Furthermore, if they add the "autocontrato" clause (self-contract, which means you sign as a seller on their behalf, and as a buyer on yours; you are sort of both parties of the contract), using the POA you could buy it for yourself in the US and change the ownership (theoretically).
"Las facultades reseñadas se confieren aunque en el ejercicio de las mismas incurriera el apoderado en alguna de las figuras jurídicas de autocontratación, múltiple representación o existieran intereses coincidentes o contrapuestos"
I hope it helps, although this is not based on Chile specifically; hopefully others will give better info.