identifying and blocking suspicious attempts

i am about to start work on a group-buying site. it works like this: a phone costs 100USD, but if 50 people buy it together , u get a price of 70USD.

so the website launches this offer开发者_如何学运维, waits for 50 people to deposit money . once the threshold is reached, everyone is sent a coupon and that person can go and get his phone.

the problem.. there will be few offers which will be time-limited. for example, total of 100 mobile phones available only. the first 100 users can only buy it. the offer starts on 5th janissary at 11am.

now, how we can detect any suspicious attempts. like a person writing a script and buying all the 100 units.

i dont think IP based limits is a good enough criteria as REAL BUYERS may access the website using the same IP.

environment is LAMP.

why should you care , the more you sell the more you make money :) if the transaction is made you can enforce that user can't buy with same payment method and credentials twice. And as you said user must deposit money before he can make the purchase. You can ask user credit card data before the sale is opened, that is rather good limit as scammer must use 50 different credit cards