Sad thing on this: If you lose a lot of troops and gear in that fight, 700 crates hardly even your losses on gear. Esp. for troops you have to keep in mind that it takes some days to recruit them back.
What kind of expensive gear are you buying?????
All you need for 400 man army to beat their peasant automatic gear is 400 gambeson quality armor with 1 gold each foot and head armor (400 each then), 10 horn bows and 5 xbows (reused over and over again), 20 short bows, 200 arrows, 100 bolts, 150 cudgels, 100 yellow practice shields, 40 falchions, 40 board shields, 100 tridents, 25 scythes or boar spears, 50 practice longswords, 25 axes.
Cost: Under 5000 gold
Your ranged will devastate their bow using asses. and you have equivalent melee gear which is all you need with their crappy armor.
Optional for additional devastating value: 20 rouncies, 20 heavy lances
Goods : 700
Price: 50-75 gold based on average distance trade
?
Profit: 30K+
If you double the number of quality gear you still have less than 10,000. The hardest 2 things are actually getting 400+ troops (good to have a small bandit clan of 2-3 friends cuts the time in half to a third) and then getting a full roster for the attack - best chance on that is to attack at prime time. Getting 1-2 people to join your cause not hat hard - talk to people in game, find people without clan tags.