How to increase merchants cash pool? It drains quickly, when selling armor and weapons.
Well..... you really can't without investing money into the merchant or getting a specific skill from the speech tree. When you complete a quest for a merchant, you can invest 500 gold, I believe, into them and they will permanently have 500 more gold from that point on. The one for the speech skill is one of the last skills you can unlock there and it adds 1000 gold to every merchant. There really isn't much else after that though
You just have to spread out the selling to different merchants all over skyrim if you don't want to do the above.
EDIT: I just checked, you actually need a skill to invest money in them
So...... yeah, if you want to do this, you need increased speech
Personally, I wouldn't waste the skill points, but it is your game so...... yeah :D (DOn't forget, there is a level cap so you can't get every perk from every skill tree!)