They could track used game sales like they track a lot of things (i.e. new game sales, tv ratings, census, etc.). They could use surveys for a large sample of consumers representing the population and ask what games they have purchased used over the last period and project that on the whole population.

Edit: just to clarify I don't think it would be worth it to keep track of I'm just saying that it could be done. 

Games like Crackdown would at least double its sales.

http://www.destructoid.com/crackdown-developer-hates-used-games-too-140057.phtml

The sales would be very inflated.

Actually, I don't think they would increase too much at all. There are a lot of people who use their old games to get new games and I'm sure if they didn't have that ability they wouldn't be able to get as many new games. In my opinion, I feel it would be about the same. The whole used thing is pretty beneficial for everyone.

Uh... did you read anything in this thread? uhm... How bout the title did you even bother to read that? 

I rate used game scene further down my moral ladder than the piracy scene. At least pirates don't earn money off someone else's back.

Used games sales are irrelevant to most discussions as money is not made on the secondary market, only on the primary (for the company that is). Also, GDP only counts first sales.

Used sales will always be higher then normal sales, but they should be as you are counting a game twice, which is also why they are irrelevant. More copyies are not being owned. The number of units sold is exactly the same, as a used game is not a unit sold (as it already was sold). Again, not relevant to discussion.