Post some detailed pictures. I have never heard of the builder but he probably hasn't heard of me either.
There are a lot of good unknown gunsmiths out there that no one has ever heard of that do some very good work. And there are some that can do a real hackjob on a good 1911. Stay away from these. Does the seller not know the gunsmith ? Who did he get the pistol from ? Did they know him. Sounds like some research is in order.
There are a lot of things to check on this pistol to see if it's a good one. Have you shot it ? How does it function ? The word "custom" is used on a lot of pistols which only had some parts changed.