I think this will be the quickest way to sort this to add a tag as an entry point and exclude anyone who's in the buyers sequence (like you have tried). And then remove the exclusion tag at the bottom of the buyers sequence and add the tag that is used as the entry point for the welcome automation.
So the critical difference being to have the tag as the entry to the welcome sequence as well. People who are in the buyers sequence will be excluded from the welcome sequence. When they complete the buyers sequence, they'll get added to the welcome sequence.
This could also happen the other way around - someone is in the welcome sequence already and they become a buyer. In this case, you'd need to pull them out of the welcome sequence from the buyers automation (you can remove someone from a sequence in an automation).