I've got some of the material you're talking about...definatly not silnylon. I think we decided it was urethane coated ripstop.
It is extremely heavy, but it is also very durable and waterproof. I field tested my DIY tarp made of the stuff for a full week a couple days ago at Camp Daniel Boone in NC. This included heavy rain everyday of the week. It also stayed very taut without SLT's and held up great in the direct sunlight or the pouring rain.
Hopefully the guy you bought from will give back some of your $$.