In the past I recommended people not attempt to open the ecu case before shipping, but I'm changing my stance on that, especially for B13 ecus. The reason I recommended that is the ecu screws are threadlocked from the factory, and will easily strip. To remove, you need to use exactly the right size screw driver (this is where a Chinese screwdriver will not work), or a the correct phillips head driver in a drill. If the screw starts to strip just leave it alone and let me remove it, I've gotten really good at removing stripped ecu screws.
Anywho, more and more B13 ecus are starting to show up damaged when I receive them. I really really don't like to work on damaged ecus, and prefer good clean cores, especially when installing a Realtime board. Don't waste the $350 on a faulty ecu.
B13 ecus typically suffer from one of two problems: burned traces typically on the ecu power supply circuit, or water damage. The burned traces are generally caused by a fault in a sensor. Error in wiring of an S-AFC or MAF are very common causes. The first place to check is the lower right corner of the ecu on the top side. Look for black marks, and if you can rub your finger on them and your finger comes back dirty you've found one. Sometimes these can be repaired, sometimes they can't. If possible, I prefer to just get a new core.
The water damage is very common on cars from up north (but not an exclusive north problem). I think this is because the floor heater vent is leaking onto the ecu. If you are from up north I would pull the ecu even on a stock car and check for signs of water damage. Common signs of water damage are white streaks on the sides of the case, or rust on the lid of the ecu. This doesn't always mean the ecu is damaged though, often times the ecu seal has worked well enough to keep the moisture out. I think there is a reason Nissan started putting the plastic covers on the B14s.
In both cases I would recommend using a fresh core if at all possible. In both cases the ecu may still seem to function and the car may still run, but I would still try to find a fresh core.
Ok, so what does this damage look like? Well, when you pop the covers (front and back), this is what your looking for (I just happen to have more water damaged ecus right now, I'll snap some pictures of burned ecus as I get them):
This is a good example of a burn mark (this is on the ignition switch power line coming into the ecu) -
And now for water damage pictures. Sometimes water damage is obvious, sometimes its just in one or two spots. Look for green/white build-up near the input pins of the connector. If its light, sometimes it can just be washed off with rubbing alcohol and a q-tip. If there is also black marks the ecu is probably trash.
One last thing, if you end up with a known bad ecu, just throw it away or at least write 'BAD' on it. Don't sell it. Thats just wrong.
....oh well i was 1 for 2 on good ecu's 
