Most of the time people who use PayPal do so because they either can't use Western Union (for me online stuff through WU doesn't work), they are either too young (need to be 18+, many of the members on our forum are fairly young) or have credit in their PayPal accounts and wish to use it.
Its more expensive because of the fee's associated with transferring it to and from accounts, and the inherent risk that is given with using PayPal for counterfeit goods.
The reason that they accept PayPal Gift and not invoice is because a lot of the members on the forum are relativley immature, and have started disputes for no reason, which led to a few of their PayPal accounts getting banned. As such, its a Gift only principle to avoid this.