My foodsaver (the black one at BB&B...I think V3460) will heatseal the poly bags that my LHBS uses for grain. The LHBS heatseals them at the store with a two-foot wide heatsealer thing (doesn't vacuum either).
I think the idea is to actually melt them though...it's melting the two sides of the bag together to seal it.
I've heatsealed a lot of non-foodsaver bags...Tostitos, potato chips, etc. The only bag that hasn't worked so far is a pretzel stick bag. It's made of really loud crinkly plastic.