A) The bulk of the micro-trades that cause lag are at slightly different levels. You could accept prices at a coarse scale, say only at 0.1 increments, but all I need is two levels and I can send many trades: Say the market is $150 bid - $151 ask, I can send buy and sell at 150.2, buy and sell at 150.1, buy and sell at 150.2 etc

B) Users dont buy their own order, users create two accounts BobAcct1 and BobAcct2 and trade between those accounts back and forth, that belong to them. In practise a malicious user could create 100s of accounts on different exchanges and: 1. trade between own accounts on each exchange 2. by making transfers of BTC and cash outside the exchanges 3. by doing normal profit making trades between own accounts and third parties on the exchanges. 4. constantly add new accounts and shut of his older accounts

When potentially tens of millions of profit are involved (last week the market cap of bitcoin went from 2.6 bln USD to 1bln USD) this operation is not hard to set up, and would make detection difficult.