I don't buy art. I like going to openings, however.
Daniel Gould produces an excellent weekly round-up of the Amsterdam arts and gallery scene. Contact him by email to be added to his list.
For art galleries as in museums, check out the fabulous newyorkamsterdam website.
For modern art: many galleries are located in the Spiegelkwartier. Some of the big names include Reflex, Delaive and Douwes. The Amsterdam Info website has a long list.
There is a Sunday art market on the Thorbeckeplein
And don't forget the mighty Affordable Art Fair at the very end of October