The IRS can deny any credit (or deduction) they want and send you a tax bill (with added penalties and interest, of course); it happens all the time. At that point, for all practical purposes, the burden of proof shifts to the taxpayer to prove they are eligible for the credit/deduction.Since proving purchase with the intent to re-sell is nearly impossible (and highly unlikely) it's a risk I'm sure some will take. It's simply not enforceable.
If they start suspecting people are reselling 3's and claiming the credit illegally, you can bet they will start taking a second look at all returns with a form 8936 that claim a VIN starting with 5YJ. It's low hanging fruit for them.