Facebook groups are your best bet for a specific model or brand.

I don't know what's there for Specialized. For Salsa and Surly there is a buy/sell group and an owners group... the buy/sell group allows no posts of non-brand parts, with the maybe exception of something like an OEM wheelset. The owners group doesn't allow buy/sell postings. It seems like a good system. For MTBTandems there's a group that allows either kind of post as long as it's MTB tandem related, and that's a small enough group it seems to work ok for lack of anyone malicious enough to test the rules.