It can almost be taken as it has to be in increments of 10, being 500 not 502. That's how I took it at least, so I can understand the confusion. Having an interface would be a lot easier (as previously mentioned) so that users could enter in their amount they want to bid and have it show the high bid/next bid amount.
But just for clarification @AnkerOfficial, are the bids needing to be done 10,20,30,40,etc or just in 10 values (120, 240,500,etc)?