I know this is an old thread but it is certainly a commonly asked question so I will post a response I gave to another traveller who asked me the same question.

Hearsay is wrong, or mostly bullshit.

I have lived in Chile for many years and I buy and sell used bikes. Including buying from tourists.

I know many people sell their foreign "temporary import" bikes and then leave the country without a word being said. I buy those bikes and import them legally through the Zona Franca.

Here are your options:

1. If you are in the Zona Franca regions (extreme north, extreme south... the latter being basically the region south of Puerto Montt) -- then both foreigners AND local chilean residents can buy used bikes. The sale is usually handled through a legalised transaction recorded as a "compraventa" ( bill of sale - essentially) or just a declaration at a Notaría. What the buyer does with that bike is then no longer a concern of the seller (e.g., whether the buyer imports the bike legally or not).

2. In a non-Zona Franca region (the rest of Chile) then legal residents may NOT buy a used bike (well, they do, clandestinely, but it's a Customs and tax-evasion violation). However, non-residents (tourists) can buy a used bike from another tourist, and the transaction is usually recorded at the Notaría, for documentation for both seller and buyer benefit. Be aware that the bike is subject to the temporary import time limit and a bike that overstays can be seized. So if you plan to buy a used bike, do check its import status. However, all you have then is a Chile-legalised bill of sale and no normal registration paper, so you might (or might not) have trouble down the line, esp in other countries.

3. If you wish to BUY and legally transfer and register a chilean-registered bike in Chile, as a tourist or otherwise, you must first obtain a tax ID, called a RUT, and the bike will need to have current tech inspection ("revisión técnica") with emissions certification, and the mandatory insurance called SOAP. Once you do the legal title transfer you will need to keep the "permiso de circulación" current. An expired permiso de circulación can result in the bike being impounded. Normally if you cannot prove that you are a legal, permanent resident of a Zona Franca then you can't buy a bike and get the tax (19%) exemption --- though occasionally people have managed that. This whole purchase issue has been discussed elsewhere so I won't 'splain it again.