Not only is the con still used on sites such as ebay but also in mags such as auto trader - and it must work else they'd stop doing it.
The con works for two reasons.
1. People are greedy. "I'll send you a cheque for £8k, you keep the £100 for your mobile phone (whatever) plus a few hundred for yourself (my dear friend) and send £500 to my shipping agent {who doesn't exist but the PO box is probably owned by the scammer}, when the cheque has cleared you can send the item to me"
Of course, the cheque will clear because it is stolen, just the one cheque from near the back of the book.
2. Banks are incredibly lax when it comes to dealing with everyone else's money.
So. Despite the fact that you have only used cheques 1 & 2 out of a book, they quite happily process payment for cheque 48 without a second look.
Come the end of the month when matey gets his bank statement, he sees cheques 1&2 - that's fine - then cheque 48 & starts getting narky with the bank. The bank look at the details & agree it is a bit naughty so refund his money & instruct your bank to take the cash from your bank.
In a lot of cases, the scammer is the genuine owner of the cheque - they end up with a full refund plus the goods.
Some shops etc are now refusing to take cheques for this reason - they say it saves time & money etc but the reality is that they have also been conned with high value purchases & then finding out later that the cheque used was 'stolen'