So I bought and built 79103 Turtle Lair Attack and noticed one of the pieces in the instructions is different from the piece included in the set.  The piece in question is the black cage-like piece pictured below:  The one in the instructions has only two pegs, but the one I have has four.

 

 

Will this have any effect on the resale value or is this just an instruction misprint?  Just curious if I should contact Walmart or Lego regarding it as it is very noticeably different and I don't want the next owner to think I'm trying to pull a fast one over on them or anything.  

It wouldn't hurt to contact Lego directly. Walmart certainly couldn't care less, and I would them not to be of any help here.

It's a common variant. It won't affect resale value if you decide to sell the set as used.

 

Lego does alter parts design as they go along, and during the course of a set's production life it might get two or three different variants of parts as they change the design and molds.

 

I know what you're thinking, "But Frog, that must make the life of a novice parts store owner like you a living hell!" You're right, it does. 

Use the customer service link for missing or damaged pieces on the shop at home website. it is located near the bottom of the page I believe. Put in the set number and a list of pieces for that set with pictures should appear. See what lego themselves list as to what should belong in the set. If it shows something different than what you received then you should be able to request the missing piece free of charge. It wouldn't hurt to send them an email either way for clarification. Something like this would drive me nuts, especially if the set in question was for future resale.

So I bought and built 79103 Turtle Lair Attack and noticed one of the pieces in the instructions is different from the piece included in the set. The piece in question is the black cage-like piece pictured below: The one in the instructions has only two pegs, but the one I have has four.

The 31010 Treehouse has exactly the same thing where the cover of the box and instructions shows the fence with a single stud on top of the ends yet the actual included physical piece features four studs across the top. It is perfectly natural and an intended alternate meant to eventually completely replace the prior form. If you check out this list, you will see the four stud fence is a regular part as of 2014 with it appearing as an alternate in a select handful of sets dating from now all the way to 2011.

In short, this replacement part is meant to be there and it should have no effect on resale value unless some buyer wants to be a nitpicky doofus. In my honest opinion, this is how the part should have been in the first place as it is simply more versatile with the four studs across compared to just the two at the ends.