I live super close to Fenway and walk by the park quite regularly. For the Red Sox, at least, I think it also depends on a whole bunch of other factors as well like time of year (in season vs out of season as well as how far along in the season, performance of the team, how "in-stock" the merch is, etc.

For example, during rare inter-league matchups against popular teams, you'll see a ton of the away team's merch. That is to say, when the Cubs came to Fenway last year, I saw a bunch of Cubs merch available. But even outside of those matchups, you can still see Yankees stuff on sale from time to time. More so now that Yankees are actually good again after their couple years of sucking. (Those years helped cool off the animosity among fans, so that now that the Yankees are good again their fair-weather fans are willing to buy their shit again).