I tried to buy a secondhand bike in Peru and found it really difficult. As said before, the number of bikes on the market is small but the other problem is that it is really hard to find a secondhand bike for sale by the original owner...they are almost always in someone else's name and then you need notary letters etc etc... can be a bit dodgey.

I ended up buying a new bike (Honda CTX 200cc... for sale now on the Hubb but in Columbia). Even then, the papers took 3 weeks to 'clear' AND when I got to the Bolivian border, I didnt have one paper (altho the Honda dealer told me they had provided me with Everything I needed) and was held up for 3 days... in the end I had to pay $140 for an agent to dummy up (forge really) the missing document and then 'import' the bike to Bolivia via a customs warehouse. That was simply because it was Peruvian registered and they have a big problem letting Peruvian verhicles over the border into Bolivia. The other borders are all easy... Peru-Ecuador, Bolivia-Chile, Chile-Peru no worries at all with Peruvian plates.

I met quite a few other riders on bikes plated in other countries (US, Argerntina, Columbia, Chile) and none of them had ANY problems getting into OR out of Peru whether they had the original ownership docs (ie bought the bike new) or an official document from a notary saying they had bought the bike second hand from another traveller somewhere... so my advice would be go for it (if you've already got yr ticks to Peru) BUT don't buy a Peruvian plated bike!! OR try to change your tickets and buy a second hand bike wherever there's the right one available. I had a ball so I'd definitely recommend the trip!!
Cheers Matt