Someone requested the first book in a series from me.  I happen to also have the second book on my shelf.  Is it ok to PM the requestor and ask if they want to request the second book too?

There are other copies in the system so they may have requested it from someone farther up the FIFO line or they may just want to see if they like the series first.  The edition on my shelf doesn't spefically say it is the second book so they might not realize that I have it.  I would love to save some postage and move a couple of items out of my house but I don't want to seem pushy.

Yes, it's okay to PM the requestor with this information. I would suggest adding a couple of points to your PM: 

Say something like "there's no pressure to order more." It's possible that the requestor might view it as a "requirement," which is not allowed. 

Give them a timeframe to respond by, so you are not left wondering what to do if you don't hear from them. Ideally you've picked the latest mailing date possible. 

Hope this helps. 

I've done this saying something like "Thank you for ordering xxxx.  I am happy to send this one book to you, but wanted to let you know I also have xxxx (book 2 in the series) on my bookshelf."  I agree give them a date to order or respond saying they don't want the other book so that you aren't left wondering what to do.  I think most people appreciate it.

I get books out of order all the time though so I might order book 1 and not want book 2 because I already have it.

It never hurts to offer. I have sent PMs like that, as well as PMs offering deals off my shelf, as well as asking if someone might want Wish Listed books that I have that are later in the series too.

On the other hand, I also often will pick up a book at the thrift store that is later in a series, and come back to PBS to order only the first one, or the early ones.

Yes, you can offer.  IME some will accept and some will not, use a deadline so you can close the transaction and mail if there is no response.

Sure, I do it all the time. If I'm ahead of them in the series, and they have thanked me for the information, I then tell them when I post others in trhe series.

Thanks for the advice I did PM her and she took the second book.  More room on my bookshelf!

I have done this a few times, and only once was there any non-positive response. The request was for book #2 in a four-book series, of which I had books 3 & 4 on my Bookshelf. The requestor got just a little bit pissy about my PM, along with telling me that they already had the next two books in the series and were trying to fill in the gap between book #1 and book #3 before starting the series.

I did this once, though it was actually the second volume of a 2-volume academic text which wasn't very clear from the PBS listing (I only noticed because I posted both at the same time, from the respective listings they seemed like unrelated titles from the same author). The requestor was very pleased that I mentioned it.

Likewise, as a requestor I would never be upset to get such a note, whether I ended up wanting the extra book(s) or not.