I assume you want a game-mechanics answer instead of some roleplay "the barwench in Daggerfall is quite the looker!" answer.

Due to the way the trade guild system works in ESO, there is no real specialisation between guilds. That said, guilds based in lowbie zones tend to have more lowbie items and guilds in endgame zones tend to have more endgame items. There are also some regional variations and fluctuations, but they aren't really noticeable unless you become an experienced trader.

More simply if you want to shop efficiently (see the greatest quantity and variety of goods with the minimal legwork), then you should just visit the main trade hubs. These will depend a little bit on your specific server since the quality of trade hubs depends on the strength of trade guilds that reside there, but on EU-PC for example the best trade hubs are:

1) Belkarth, Craglorn
2) Rawl'Kha, Reaper's March
3) Mournhold, Deshaan
4) Eldenroot, Grahtwood
5) Wayrest, Stormhaven
6) Orsinium, Wrothgar

As for farming flowers... Cold Harbour is apparently the best... but I don't farm, so I wouldn't know specifics.

Oh and in general, NPC vendors have almost nothing of worth to buy (except AP vendors in Cyrodil).