sorry for the late response...
anyway... this is what I bought on ebay: Serial 13.56MHZ RFID Reader/Writer Module Kits
Here the documents I worked with
Manual
Test code
When I first connected the RFID reader, the State and the Mode LED were on:
(There's 3 smd leds in the bottom left corner).
When the mode led is on it means that it's in "compact command" mode, which means it expects 1 byte. So I sent 0x02 and the module responded by sending back the serial number of the tag when passed over the the reader.
I then also sent some "basic commands" like to initialize the cards wallet, deduct from the wallet and so on.
But now the mode led is always off. Which would mean it's still in basic command mode. But no matter what command I send now it returns back "FF" immediately.
From the manual:
"For any wrong command sending to this module, 0xFF will be returned."
It looks like it's stuck in basic command mode...