Wasn't really sure if this should go in the unlock section, but it seems more appropriate here since it shouldn't even need unlocking.

I bought a new iPhone 3G off eBay with the 4.2.1 f/w installed a few days ago from a reliable seller (just clarifying that). When I got it, I immediately did a restore (no shift click, just straight back to 4.2.1) as it was a named iPhone with a couple of music files on there. When it came back, it gave me the "no SIM card installed" error so I could only slide for emergency calls and the associated error on iTunes that doesn't let me do anything. I ran out of ideas and tried to put it in DFU mode so I could restore to 3.1.3 instead, which didn't work at all (1015 error), then back to 4.2.1, and got the same error.

I'm not remotely experienced on how to fix these problems but I've pretty much got no ideas and I'm completely stuck, so if anyone can help me I'd really appreciate it.

Also, this phone is locked to the o2 network and I've been using a giffgaff SIM on it (giffgaff runs on the same network, I've used them on o2 phones before). I initially thought it might be a problem with my SIM so I tried 2 others which also didn't work.