Newsgroups are great for reading and exchanging ideas, but for doing good research, you need to follow up critically on the ideas you get. You'll meet some pretty terrific and helpful people, though. I want to sell something. No, you should not post information about things you are selling, unless you are posting to the soc.genealogy.marketplace. This group is intended for advertising about goods and services relevant to genealogy.
Advertising is considered off-topic in the other groups.
I read a message last month and now it's gone. It is probably gone. Messages are stored on individual news servers for varying lengths of time, depending on how much disk space the server has and on how many newsgroups they carry. Most sites will keep messages from a few days up to a few weeks. If you see a message that interests you, save a copy to your disk.