Can you still get the $8,000 tax credit by, buying a house for sale by owner?

The method of sale is not relevant. As long as you qualify on all counts (not buying from a relative, 1st time homebuyer for all parties, etc.), and the IRS accepts a mobile home as a residence (which it has in the past), then you are fine.

Just as a warning, the tax credit is for 10% of the purchase price of the home or $8,000, whichever is less, so if the mobile home is, say, $25,000, you will only get $2,500.

only if it is deeded as real estate. Homes in parks in most cases are just chattel

As long as the deed is in your name and you did not buy it from a relative you will qualify. You do not have to have a mortgage, use an agent, etc.