Based it on the in-person Heath's we obtained when he was alive. I am traveling and don't keep my scans on the laptop or I would post some examples for you. Its much easier to have you look at an authentic autograph than to try and explain it to you.
If you search this forum there is a recent discussion with some authentic examples posted to look at. That should give you an idea about his signature.
The price alone though should be a tip off. If it was authentic somebody would have snapped it up and the seller would not have needed to put it 'on sale'.