just read ebay info looked at the zip for me the only issue with zip I can see (letting water in. asside) might be as quidditys pointed out insider access?
Form follows function. Its basically a very large full zip holdall, which is designed to be fully waterproof to stop fluids from decomp, blood etc from making a mess everywhere. It tends to lie down flat so the body can be easily moved into it, and then zipped up. Firstly, the thing is designed to be waterproof, but since a cadaver dies not breath, has no need to be breathable, unlike the living. Next, the cadaver isn't trying to get out of the bag (although there is a really good bit in the original
The Fog...), so the zip access is limited to access from the outside (is on top, so not perfect if it rains), and obviously there is no need for a hole for your head or to breath. There was that chap last year who was found in 110L TNF duffle, but that was not good, and its not something most people would want to try.
If you want to have a look at a modern bag, google 'disaster bag' - you wont find it on a bushcraft or hiking website....
Apart from the fact that its really ghoulish, that its completely unsuitable for a bivi bag, and you can get the real thing for about the same price without any extra hassle, what could go wrong? If I was a farmer having to get a dead sheep in the back of a landrover with the minimum of mess, or someone who'd just shot a small deer, etc - fine. Otherwise, no.
Seriously, its a terrible idea. If you want a bivvie, head over to Endicotts (or ring Kev for more advice) or get the Alpkit Hulka for £35. If you want a ground sheet, get a ground sheet. Go Outdoors will do you a 13 x 10 one for a tenner, and a 6 x 4 one even less. Repurposing is one thing, but why make life (or death) harder than it is