I wanted 10190 Market Street to complete my modular collection but at $500-600 USED! there was no way i could justify buying it. So i decided to build it on brick link instead, however after pricing everything out on a massive excel spread sheet i realized it was stilling going to cost nearly $250. So instead i built two of them, when finished i sold one of them for $500 and used the profits to cover the cost of the other building, so basically i got a MS for free + my time.
Time...ah yes, it took me nearly 3 months to complete them, and i ordered from 17 different stores, it was a massive under taking. While i did make $500 (the value of the MS i got to keep) i would guess i spent close to 50+ hours working on it, maybe more, at best i was making minimum wage for my effort. Also, there were certain pieces that were very difficult to find enough of on all of bricklink to build two MS, if i would have been trying to build 3 i wouldnt have been able to do it.
So i would say yes you could make money off of doing this but not as much as you think and it will be more difficult than you can ever possible imagine.