zifank: can you give me a sample code for controling a multi seven segment 7 digit?
Yes, I could easily give you some example code. But it would be pointless. I know it would not work. Code has to be written to exactly match the hardware. I have never seen your hardware before.
If you can get no help from the seller, you can do some "reverse engineering". Identify the components from their part numbers. Trace the tracks on the PCB. Make your own schematic diagram. From that, you can write some code. We can help with some stages of that process. But you have to start, because you have the hardware in front of you.
Looking closely at the board, this is what I believe I can see and guess. There are 4 chips. It would make sense if they are 74xx595 shift registers as you say. The chip closet to the connector drives 6 transistors which in turn drive the commons for the digits in that column. The other 3 shift registers drive the segments for the digits in the 3 rows, via current limiting resistors. The 4 shift registers must be "daisy-chained", given there are only 5 pins on the connector, including power and ground. I would guess that S=Serial data, C=Clock and R=latch.
The code must perform the multiplexing, sending 3 bytes of segment data and 1 byte to select the digit column, and lighting one column of digits at a time. The most important answers you need to find out, I believe, are the digits common anode or common cathode and are the transistors NPN or PNP (or are they FETs). The part numbers written on the displays and transistors may help answer those questions.