Be VERY careful at the used shops downtown. Not to talk bad about anyone in particular, but there are two used shops right next to each other on Bromfield St in Downtown crossing. One is decent, the other shop employs a few swindlers. The biggest used selection is at EP Levines. Their prices are weird, sometimes a steal, sometimes a laugh. It's not downtown though. You would have to take a bus or taxi to 23 Drydock Avenue from South Station, or walk a mile or two down Summer St, across a bridge, into what I believe is technically South Boston. EP Levines is one of the places with A LOT of used stuff behind glass cases, and a lot more in the back. The also have a rack of used enlargers, lights, etc. If you have time you should check it out.