First go to the direct sales. Select these options
Price: = 500e
Pass: = 1
BM: Morpheus Arms
Gender: Male(NOT gelding!)
Pure breed: Yes
Buy the highest gp stallion you can find
Offer covers for 1000-2000. Do not go over 2500e, this will make the covers harder to be accepted.
Keep doing that until your stallion gets to age 30, then kill him. You'll get your pass back.
To get 5 covers in a day:
Meadow for forage
Feed grain and energy mash
Box until 20:00
Train or beach ride or whatever until the time is 22:00 or the energy is 20%
I sent out about 80 covers, and within 3 days of sending them out, I made 400k. It took only 5 horse years to send that many covers so if you buy a stallion at age ten you could probably make 1,000,000e by age 30.