Why bother? I try for 500-1200 bushels normally. I have 8 caravans with me right now.
"-I can save your city from starvation good Sir. Even though I only have 50 bushels with me and I promised them to the Lord of Kepler, if you give me 100 pieces of gold I will leave them here and tell Lord Kepler I lost them to bandits.
-Yes, do so!
Three days later:
-The city is starving again!
-You are in luck, after three days of relentless search, I could find 50 bushels more!"
Lather, rinse, repeat. I carried 700 bushels with me for more than a year, never saw a need to sell it all at once.