Do these senior members actually know what they're talking about or are they senior members simply because they back every post they see with the goal of gaining activity and merit?
No one can give an accurate answer, unless you provide their names.
I went through a history of one member and found that the majority of post they have been posting since they became a member were accusations of scams and trolling on the same threads.
You should report those (trolling is against the rules).
This leaves me thinking that the BTT ranking system encourages users to troll and create spam
No, read the following posts:
3. No trolling.
However, trolling isn't allowed. If a user is habitually posting obviously false nonsense ("obviously false nonsense" to an outsider, NOT to someone who follows or is involved in the discussion) just to stir up trouble, then it's considered trolling, which is prohibited. Such cases should be thoroughly documented in the report though (There are tons of reports that just say "trolling", but moderators don't have time to look through each user's post).
and that rank on BTT really doesn't mean anything in terms of someones credibility.
Correct, especially since there's the issue of "account sales".