I am having a marble shower installed, but the drain pipe is off by an inch in relation to where the drain cut was made on the marble. How hard will it be to move the plastic drain pipe over an inch? How would I go about doing that?

There are several ways to do this. The more of the drain line you have access to, the easier it will be. Do you have access to the trap? I have used a flexible trap in a sink drain to solve the problem of adding one originally left out by somebody else. If you have access to a horizontal line, you can cut out or add a short length using a coupling or 2. To go sideways a little, just figure out how far back you need to install a 45 or 30 degree ell. If you just have a short vertical section, you may be able to cut it off and add a couple of 30 degree elbows and a short length of pipe. Try to avoid 90 degree elbows and use long radius drain elbows. You may even be a able to replace an existing 90 with two 45's and turn things until they line up. Put things together dry and make sure they fit before putting cement on the joints. I know the common PVC is easy to work with and likely the less common ABS. PVC will also bend a little to accommodate the last bit of misalignment.

Haven't seen much of Tom for a while. Maybe he will pop in with a better idea.

It will depend upon how open the trap and trap raiser are and how long the trap raiser is.  If the raiser just sticks up above the cement and is cemented in you will have to break up the cement and open up the area.  If you have space enough between the trap and the floor to install a street 45 into a regular 45 and create a 1 inch off set on the vertical raiser that would be easiest.  If the trap is so close that you have no space to offset the raiser then you will have to move the trap.  This is done by breaking the cement from around the trap back along the horizontal discharge line and cutting in a 45 to move the trap and raiser where you want it.  Of course the easiest way by far, (if the marble base was custom build)  would be to say, "Hey guys!  Ya missed the hole,  try again."  However if the base is a "ready made" then the options I described  above would apply.   I think you should be aware that PVC fitting joints are tapered.  If you put them together dry without first priming and applying glue they size up and you will have a hard time pulling them apart.Good luck,  Tom