I don't have a great answer for you, but may be able to point you in the right direction. In economics the market for used goods is known as a "secondary market." This is often used in relation to financial instruments, where the stock market that you trade on is the secondary market, while the primary market is the market for initial public offerings (IPOs) and follow-on offerings. In finance it is widely believed (and I assume there is some support, although I haven't looked specifically) that the a secondary market for a financial instrument is an important factor in establishing it's primary market price, but a healthy secondary market in this case actually serves to increase the price in the primary market, because purchasers will pay a premium for higher liquidity. For example, when a corporation issues new bonds, investors will consider how actively traded the corporations existing shares are when determining their willingness to pay. I assume that for physical goods the dynamics of the secondary market have a very different impact on the primary market, but a search for "secondary markets" on google scholar should probably turn up a starting point.

Well, one way is to look at shocks to reselling or buying used (new regulation, DRM, etc.) and see how much demand for new products decreases because of the de facto price increase. Another way might be to think of reselling as a subsidy to buying new - a rebate, if you will. If you can estimate how much a discount would increase sales and the producers' profits, then you can take the average resell price times probability someone will resell and treat that as a discount and figure out from there. I know I've read economics research on this topic in the past, so you can probably google up some good papers.