Target Plural or Singular words???

[Monsieur]

Originally Posted by SaucyPanties

I have found panty and panties are the same when it comes to search engines

Sorry, but that's wrong!
Search engines can differentiate between the singular and plural of a word (just look at the SERPs for "panty" and "panties" on the big engines), so you should choose which one to put more weight on.

You chose "used panties" which I think was a very good choice, because it should get more searches as "used panty" (but I didn't look that up, so can't say for sure).

While writing the copy for your web page you should always use the word which fits the sentence (as GreenGuy stated), but you should've chosen the keywords you want to optimize for before starting to write the copy ;-)

So if you want to target "used panty" you should think about phrasing the sentences you want to use so that you can write "used panty".

To the initial question:
You should optimize for the one that
a) gets the most searches or
b) has the better search/result ratio
depending on your seo skills.

If you're sure you can achieve top positions for every keyword you target, then of course you should go for option a). But if you're just a small webmaster, you should go with option b) just because that's how you'ld end with better chances to get good listings ;-)