Go to Hog and Cogs on Staunton Island. (the middle one)
if you dont know where it is on the island, it is just north of the curved enterance to the bridge to the leftmost island. (the middle part of the island to the west)
do the missions (you need to do about 10 of each bike)
The freeways are a pain because i often got wanted when i sold them because you have to hit the sidewalkers, i was only able to do like 2-3 at a time because i got wasted or wanted. if you can get them all in one selling period then more power to you.
then the money icon will show up. Im pretty sure it maxes at $3k so get it often.
After you have finished all the bikes, do the mission some more. only hop on the f@ggio (site censor) because it is the quickest to sell. (I just went in a small loop and the bar goes right on up.)
Each one you sell earns from $300 to $500 and 110 seconds.
every interval of 5 you get the following bonuses:
05 = $2500
10 = $5000
15 = $7500
20 = $10000
25 = $12500
30 = $15000
35 = $17500
40 = $20000
45 = $22500
50 = $25000
55 = $27500
60 = $30000
65 = $32500
70 = $35000
75 = $37500
80 = $40000
85 = $42500
90 = $45000
95 = $47500
100 = $50000
(sadly there wasnt a bigger bonus margin by 100)
doing this, getting an average of $350 per commission, you would get $551000 by selling 100 bikes. not bad eh?
($146000 for 50 sells)
I would assume it works with the cars on Portland Island too but i just like motorcycles better :P
Note: there is often avenging angels right outside hog and cogs if you want to start those missions too. (i believe the Staunton Island reward is that you dont have to pay when you get busted.)