It's not perfect, but a decent rule of thumb for identifying buyers vs. sellers is playoff odds above/below 10%ish
Currently the teams below that line are
AL - Royals, A's, Angels
NL - Nats, Mets, Reds, Giants, Rockies
The Padres are really close (12%) but AJ Preller is a sociopath so I wouldn't expect them to sell unless they really tank the next few weeks.
The O's, Jays, Tigers, Astros, and Dbacks are also under 30%, so a bad two weeks would likely tip their scales towards selling.