Yeah, that's an offset for a 3 series. (the ET35 part) The correct offset for an E34 should be around 15. The offset relates to how the wheel center relates to the hub. ET 35 = 35mm towards fender well, ET 15 = only 15mm towards fenderwell. You could mount those wheels with a 20mm spacer, but I hear that is not recommended.
Don't worry, you're not the only one to get burned lately. I won an ebay auction over a month ago for some racing dynamics wheels and tires, they were listed to fit E32, E34, and E38... also E36 with spacers. So that sounded right, but the tires were 215's... so emailed the guy (after I bid, unfortunately)... about the offset, he said he thought it was 15. Some sketchy communication and a month later I get the wheels and tires... ET38 is the offset... I was so pissed. Plus, the tires weren't as listed. 2 wheels mounted with Nittos, 1 with a sumitomo, and 1 with a 205/50 goodyear. Ridiculous. I hope you didn't spend too much, but those wheels look nice... you should be able to sell them to a 3 series owner easy enough. That's what I'm planning on doing anyway (in a much more honest fashion that I recieved, for sure.)