Theymos can encourage whatever he wants, but he never set any written rules down regarding the tagging of members.  The community might have a problem with someone who consistently leaves negs for petty reasons, but Theymos specifically said that negging account sellers was alright--and no, I don't have a link, but I asked that question specifically.

As to Groundbit and whether he should be tagged, my question would be when the last time he bought or sold an account was.  If it was in 2018, I wouldn't be inclined to take action.  If it was in the last year or so, hell yeah I'd give him negative trust.

I also don't know what his motivations were in creating that B&H thread, but regardless of whether he himself has sold accounts in the past he isn't wrong in his advice about doing so.