If you say 'xx is welcome' it means that you are happy to receive something, for example 'Any feedback is welcome'. I'm not sure if this quite fits your situation. Are you saying that you are happy for other people to repost your posts? If so you could say: 'You are welcome to repost ( this information)' or 'Feel free to repost ( this information)'

"Reposts are welcome" can be used, but sounds a bit awkward. I think most people would use the language of the social media platforms, "reshares are welcome".

If you are asking permission to repost someones post then I'd probably say: "Hi, may I repost your article?" -or- "Is reposting welcomed?" However, I think the first option would probably be the best.