Looking at this and similar sites, the two problems students face when looking to buy an essay are:

1) The essay itself has been plagiarised, so the student can get caught out for plagiarising even if they did not intend to do so. Lavinia's advice on this is probably the only practical advice here, other than 'find a reputable site', which is easier said than done.

2) The student pays up front and has no effective redress if the essay provided is sub-standard, late etc. Effectively, the student carries the risk, which no doubt deters lots of people from using these services. In most areas of business, the supplier takes this risk and there is good evidence that most customers are honest (see the example in Freakonomics about the 'honesty box').

Why do companies in this sector not do the same? For me, this is not an academic question, as I have just set up a website offering exactly this service - payment on satisfactory completion for smaller jobs; ditto for larger jobs, but with an interim payment on proof that substantial work has already been completed. No doubt someone will accuse me of shameless plugging and breaking this site's rules, but I believe that what I am offering is unique. (In case anyone is interested, the service I offer is very specific - essays on modern history, with the UK market in mind - and my website is historyhomework.net.)