Simple Solution:
Any faction over 10 members automatically gets a banner for a monthly fee. Larger the faction, larger the fee.
Any faction or person under 10 members wants a banner? Buy one of the remaining banners slots on the market auction.
Main problem isnt the gold nor the singleplayer buying slots.
The problem is that major clans with high player count can actually lose their slot and have to play w/o it.
Instead of giving the once auctioned slot free for all, give clans the possibility to keep it for a fee. 1 week before it runs out, make a pop up message for all members like "Banner slot about to run out! Want to keep it? Deposit 100k gold in clan bank and press "yes""
Something like that... that way inactive clans gonna lose their slot and active players can keep theirs.