I have only read a little Tanith Lee, she had some really interesting ideas. I really need to read more.
I think saying his work is a ' rip off' is a little bit of a stretch. Having a pale goth looking MC is not a unique idea to anyone. I think it it is also likely that some of Gaimans own appearance at the time went into the design , pale, tall, likes wearing black and with wild dark hair.
He writes a lot about mythology and gods, again having a same theme is not a rip off of another writer.
Sandman used a lot of different artists over its run. Different stories and chapters look very different because of this, it is one of the neat things about the comic imo. Gaiman is not an artist, he just wrote the story and scripts.
I think, if there were more clear evidence of copying, it would have been picked up by now.
When a writer/creator does something awful, people always rush to try and say ' well they were not that good, they copied xyz....' to try and help, but it is always a big reach. Like people who say Harry Potter is a rip off of The Worst Witch as they both take place at a magic school....because only one person is ever allowed to use a setting or theme.