Step 1: Take the SD card from the RaspberryPI and put it in your PC.
Step 2: Download Win32DiskImager, Point to your SD card and click READ. This will make a BACKUP of your SD card. You can currupt the hell out of it by shutting down incorrectly, presonaly I run my builds from USB flash drives as they are far more stable.
Step 3: Now that you have your backup, put the car back into the Raspberry.
Now, this is the issue with buying something (premade), we dont know if they jacked with the menus, settings etc. But I will assume they didnt.
The first step is to see what emulator core the other systems are using, I find the libereto the easiest to configure as there is a controller menu. It SHOULD display what emulator it is using, or ask you to press any key once a game launches. If it doesnt, someone messed with the runcommand.sh.
If you do this, see what core its using and follow the wiki to configure the controls for that core. If it is liberato by chance then we can go to the next step
Step 4: Is there a display at the bottom when you load a game that shows what controller configuration it is using? If so, connect your Raspberry pi to your routher and SSH into it via putty or connect to its samba shares via \\RETROPIE\ from your windows explorer.
Go into configurations and find your controller CFG files, you can edit them with a text editior. From there you can assign your buttons.
If it is a Xarcade, it uses MAME standard confgs like 5 , 6 for coin and 1 and 2 for P1 and P2 start.
You may find that pressing COIN 1 and (SOMEOTHER) button bring up the Retroarch config menu.. This will allow you to create \ save emulator \ game specific key remap files.
Of course if this seems daunting, you can always upgrade to the latest version using the setup scripts.
You can also start fresh copy the ROMS and BIOS off first through \\RETROPIE and start with a new image, configure it yourself so you understand what is going on and then put the roms back on.
If all else fails you can always restore from your backup you made earlier.
Does the person you bought it from provide support?