Create 2 accounts. 1 sell to the other non existent items until reaching more than 100 evaluations (which is done right after one item is sold).
Why do they do it? Because when a seller reaches 100 sold items, he/she receives the money from the sell immediately. So the scam is worth the effort because all the work that he/she had to make all the fake transactions since he/she will immediately get the money from the sales now.
Also they won’t have to pay transaction fees because anyone can activate the 30-days-trial sellers’ club.
Now DO CHECK those two VERY RECENTLY CREATED salesperson’s history (they have done 100 completed transactions with each other and very recently added over fifty VERY WANTED ITEMS FOR CHEAP):