Picking and choosing your clients is another thing Bernie madoff liked to do too! Analysing the types of people who are putting money in, is essential for the Ponzi to keep going. You need gullible people who believe these sorts of returns are realistic to begin with, then you need them to believe you're actually trading.
If you're not familiar with stocks/investment (which I imagine many people are not, which is one of the biggest reasons scams like this work) you're a prime and easy target to think these sort of returns are realistic. Bernie also turned down clients to look for client's with much higher capital to invest (small capital investments was more management and eventually they would need paying out) it is entirely possible declan was also doing this to look for client's who had a large sum of money, so he could feed other client's the scraps of this investment.