How does one go about buying property from the city? Such as an old building the city uses to park plows.

I'd first double check who legally owns it via the assessors database. 

https://www.cityofboston.gov/assessing/search/

Usually if its run down or an abandoned plot its owned by the DND (Department of Neighborhood Development). Because they often sell at a discounted rate they also often have specific terms for development or you have to first send in a proposal. 

https://www.boston.gov/departments/neighborhood-development/how-we-sell-citys-land-and-buildings