Hey, Aertew!
Here is how it goes.
1 As a rule, Trade guidls DO have a weekly fee because it's a need to pay for the trader (they cost millions of gold per week).
2 Traders differ. The ones in big cities, Wayrest, Mournhold, Vivec, Rawl'kha etc, have a high-traffic, lots of people pass and go by, surf those traders. Thanks to this, your stuff on sale will go pretty fast, you'll make gold quicker. They cost really... like millions of gold. (so the fee in these guilds may vary from 2k per week up to 10k per week). Traders in small villages or on the roads are not so frequently visited by potential buyers, so they cost less. Still, the GMs of guilds who hire them are not that rich to pay themselves for everything. so in the guilds with such traders also might be some fee, round 1-2k\week.
3 Social guilds, thanks to a number of players (100-500) may afford a guild trader, again, not in a high-traffic spot. Besides, small new trade guilds that are just growing, can have a simple trader... on the road someplace....) there, you might find your simple way of trade. With no fee at all ;P
Take Care! Hope that helps
Yours,
Higuchi the Mage
P.S. I can invite you to a guild with a high-traffic trader, 2k per week