I did my three OKW add-ons directly through Disney because it was much simpler. I didn't want to wait to have the seller accept my bid, wait on Disney's ROFR, and then pay closing costs. When I did my add-ons the relatively small savings wasn't worth the hassle on such small contracts.
There are advantages to going through resale brokers, but there are advantages to going throguh Disney as well, each buyer need to base that decision on what works best for them, and what they are comfortable with For larger add-ons, where the savings could be substantial, I'd likely go the resale route.