The only documentation I'm aware of that explains the how to make a call to the Trading API is [here](
http://developer.ebay.com/DevZone/XML/docs/Concepts/MakingACall.html). (Which you may already know about). You can only use HTTP POST requests and the body of the request will be either XML or SOAP. Calling the Trading API via REST style requests isn't possible as far as I know.
That's correct - there's no REST calling of the Trading API, just HTTP POSTs. eBay removed that functionality back in around 2008 or so. (Why did they remove REST from Trading? I'm guessing it's because the Shopping API was launched and stable by then, with REST capabilities, and it might've simplified their maintenance/management of the Trading API to not have to handle incoming REST calls. ...Just my $0.02 there.)