Why not ask him follow up questions? Based on my reading of his ad (not the Unicom site), it appears that there might not be a monthly fee at all (simply draw down on 1G until it's gone or six months pass). While I realize that most of us pay monthly fees, telecom products are always being packaged in new/different ways, so anything is possible.

Frankly, in light of the seller's incredible ebay rating, I'd be inclined to just grab it and sort out the details once I arrived in China. If it turns out there is no monthly fee and you don't use 800M+, you'll probably be able to unload it for the same $20 before you return home. If there is a monthly fee and you use it for one month, your total out of pocket would be $35 and you could presumably fetch y100 for the partially used card. A great deal in either case.