Well, I finally found a decent used mountain bike, and thought I'd share my experiences of finding it with others on a similar hunt:
As evidenced by the lack of replies to this thread, there are no used bike shops in B.A. The new bike shops occasionally have one or two used bikes, but never more. I'm still trying to figure out where all the used bikes that one sees on the street are bought and sold. Perhaps Mercado Libre, which I had been trying to avoid, but is really not that much more of a hassle than trucking out to each of the bike shops to ask if they have any used ones in.
MizzMarr - with all due respect, it's definitely possible to find a decent used mountain bike under $1000 pesos. None of the ones I looked at were over $550. They were all 21-speed, mountain-bike style frame, handlebars, seat.. some with shocks, some without. No name brands, but I didn't really expect nor need that.
I'll list here some of the shops I visited in my quest, ordered by neighborhood:
around Parque Centenario: the shop called Nodari that MizzMarr mentions above is medium-sized.. they had two decent used bikes as of last week, one better than the other. They both needed a bit of a cleaning / tune-up. The better of the two was going for $450.
There is another bike shop not far away, on Díaz Vélez and Acoyte I believe. There was a line out the door, so i didn't wait around.. they look big enough that they might have used bikes.
Almagro: on Bulnes, just below Sarmiento, facing the park. This one is closed from 1-3pm, so I wasn't able to check out what they had. It looks pretty small.
Colegiales: on Ciudad de la Paz, between Lacroze and Teodoro Garcia. This is a huge shop - good for buying all your accessories - helmet, lock, etc. Be warned, they're closed from 2-3pm each day. But open until 8:30pm. They didn't have any used bikes when I asked and were not particularly friendly about it. But it looks like they probably do good repair work. They do have brand new "playeras" (beach bikes - upright handlebars, foot brake, no gears) for $450, which I thought was quite reasonable.
Also in Colegiales - at Superí and Virrey Olaguer - totally a neighborhood shop, run by a father & son. Nice folks, a bit disorganized, they mainly do repairs. Reasonably priced, although their repairs are not particularly thorough. They sell some new bikes (playeras again for $450, and "full damas" for $520. Love the name - picture a classic English bike for ladies to ride on with full skirts - they have a basket in the front, chain guard, fenders, the works). This shop sometimes has used bikes, but nothing at the moment. They have a lot of kid's bikes, for you parents.
Palermo / Colegiales / Chacarita: at Cordoba and Caranza. Small shop, looks like they probably have used bikes. In fact all of them might be used, judging by the place. Like stepping back in time. Never found out though, as I got tired of waiting for the one old woman taking 20 minutes to serve the guy in front of me.
Barracas / San Telmo - at Defensa and Caseros, by Parque Lezama - a medium/large shop, they had one used bike when I visited a week or so ago. I almost bought it for $450. It was clean, good components (Shimano shifters), good tires, true wheels, but there might have been something wrong with the rear hub - it stuck a bit, didn't spin totally freely. They're open 9am - 10pm.
I'm posting all this in the hopes that it inspires others to list the bike shops in their neighborhoods - whether they sell used bikes, if they offer reasonably priced (and quality) repair services, etc. Us bikers have to stick together!
Oh - bike helmets are going for $70-120 pesos. Locks sell for $45-55 for either a U-lock or heavy chain-type.
What I found ultimately is that like everything else, whether it be furniture or electronics, you'll get the best deal by buying from a foreigner about to leave the country. They're not interested in making money, just in getting rid of their stuff. These are usually advertised on Craig's List, but you have to have patience - they only come up once every month or so.
Now a question back - anyone know where I can buy bungee cords? I have a bike rack on the back, but need some bungees to strap things to it.
