There are many arguments against the tax credit. One argument we make is that the tax credit is a benefit for sellers of houses more than for buyers of houses. This is simplest to see if you imagine a permanent credit available for all buyers: “Imagine the credit were expanded to all home buyers and made permanent. This would simply boost housing prices at the low end of the market by close to $8,000, since all buyers would be willing to pay $8,000 more. (Prices would rise by a little less than $8,000 because at higher prices, more people would be willing to sell.)”
Small point: Nemo (in a follow-up post) says that the tax credit should boost prices by exactly $8,000 (leaving aside leverage for now), because in the short term the supply curve is vertical. I’m not convinced. The reason we said “close to $8,000″ is that the supply curve is typically upward-sloping, not vertical, as shown on the graph in that follow-up post. The supply of houses can shift quickly, because people can decide to sell their houses (say, retirees planning to move to apartments in the city can move that decision forward). Also, if the credit is not available to everyone, it won’t shift the demand curve by exactly $8,000 at every point, because the demand curve for houses is the sum of every individual’s demand for houses, so only some people’s demand will change. This is why expanding the tax credit to everyone is such a bad idea. When you restrict it to first-time homebuyers, they get at least some of the benefit.
Bigger point: Nemo points out that the $8,000 increases the homebuyer’s ability to make a down payment; since mortgages provide leverage, this means the potential impact on prices is much higher. If you are buying a house with 3.5% down, then arguably an extra $8,000 in cash (which some states will advance you) can boost your buying power by $200,000. Now, this is a complicated issue, since unless you can get a no-doc loan, you still need to qualify for the monthly payments. (Nemo discusses this here.) But I think it’s fair to say that at least some buyers are constrained by the down payment more than by the monthly payments, especially with interest rates so low (I saw this in my summer legal services job). So the potential impact on a household’s buying power could be a lot more than $8,000, as Nemo says.
The net effect is that the buyer pays an inflated price for a house, which will get deflated when the tax credit prop gets taken away. I believe in some places you can effectively use the tax credit as your down payment; this means you will have close to zero equity when the credit goes away, unless housing prices rise.