I actually did this before for a client in the auction market. Most of the initial users were in the Texas / Louisiana area. After around 30,000 users we had a nice critical mass.
The biggest problem came from the client not anticipating the support required fro 30k users. Fraudulent transactions like fake Rolexs with western union money transfers were too common. Another issue that came up was people paying for postings yet using stolen credit cards. If you charged $5 to signup, but then had a chargeback, you would get dinged for that $5 and a chargeback fee of $25.
The biggest problem came from the client not anticipating the support required fro 30k users. Fraudulent transactions like fake Rolexs with western union money transfers were too common. Another issue that came up was people paying for postings yet using stolen credit cards. If you charged $5 to signup, but then had a chargeback, you would get dinged for that $5 and a chargeback fee of $25.