- Yes it's possible, the java sources are out there and could be downloaded (and afaik, the jar is not dexed until you compile your application) - see some kind of manual over there at developers.android.com to read about how to get the sources
- I've heard about a PHP project trying to attempt it, but using java should be no problem at all - you do the same things on the server, the manuals state for the client - and let the client sends the encrypted response from the market directly to the server via SSL (e.g. byte[] post)
- You should get a UID from ResponseData.userId
Read some more in this blog post and more info and links in another SO answer