We shouldn't have bought this stuff, we don't need it.

corrected:This is useless equipment, tools, etc. If I say; corrected: Did we buy these for nothing? or Did we buy these in vain? Are they OK?, If not, what would you say?

The word "stuff" sounds informal, and "in vain" sounds formal. "Did we buy this stuff for nothing?" sounds OK, but "Did we buy this stuff in vain?" sounds strange. I probably wouldn't say either (buy for nothing sounds odd). Jerry's answer "We shouldn't have bought this stuff" sounds OK. Or you might say "We wasted our money on this stuff."