Half of the teams don't even have merchandise until mid-season of a given year (meaning they'd have nothing to put in the tent for half the season, yet would probably be charged for a whole year), and the other half are unlikely to see the point of combining forces. In any case, I think they get most of their merchandise sales online nowadays; the on-circuit part of the deal simply isn't as big as it used to be, mostly being occupied by people who don't order things via the Internet, people wanting a souvenir of the day or parents trying to encourage young fans to make a connection to the sport (all important, but none the main source of income from merchandise). Impatient fans definitely don't go to large halls nowadays; they get their phones out and order without having to leave their seat at all, assuming they didn't run out of patience at their computers on an average Tuesday four months earlier
This is especially likely to affect the very "send home via DHL" items that Liberty were hoping to be able to add to the repertoire. Bernie wanted a one-stop shop too, but funnily enough he couldn't ever convince the teams to sign up either (though back then, it was more to do with Bernie wanting too much money rather than market forces making the idea fail).
Besides, engaging the group who came up with the failed NASCAR version*, which from what I gather was difficult to use despite all those tills and didn't help anyone's sales, isn't promising in terms of F1 getting a solution that will make Liberty happy (in that, even if it got its idea, it probably wouldn't get its money increased... which was probably the point...)
* - I could not have used the NASCAR version as described; it simply would have been too noisy and busy, if it had been crowded enough to make more money than a typical individual-sized, open-air, single-till stall.