Possible incorrect receptacle wiring
I recently completed a basement renovation. During the renovation, my bathroom and outside outlets stopped working. The electricians checked out the issue before leaving and found loose wires in one of my bathroom electrical outlets, which they fixed. They also told my general contractor that all the outlets on my top floor were faulty, due to poor DIY installation by the previous owner (I just moved into this house). They offered to fix the outlets, plus install GFCIs in my bathrooms for $800. I opened up one of the outlets, to see what was wrong. The wires seemed to be connected correctly (I googled...). I didn't want to open up every outlet, so I bought a tester and tested the rest. They all tested fine. I don't want to conclude that I was mis-lead. Are there any other issues with wiring that are within the realm of DIY that could be wrong with the wiring that the receptacle tester wouldn't pick up? Should I have a different electrician look at it?