"How safe is it to buy a pre owned home built kit plane? A factory built feels more safe, but is that gut feeling justified?"
Zero difference provided you get qualified people to inspect the aircraft who know the specific type. In the example given above the AA5B had been imported from France & given a UK C of A then sold by a reputable company. When we got it striped for paint a repair showed up which was not up to standard. The co that imported it paid for the work, which would have run into many £1000's, so we were very lucky. The cost of any such mistake on an LAA machine would of course have been much lower.
Rod1