I've seen people offering VHS tapes and such free with an order in the Book Bazaar, so it seems like it might be allowed.  The thing to remember though is that the yoga pants would disqualify you from sending the package via Media Mail, so you might want to consider the cost of mailing before making the offer.

If you are considering giving things like yoga pants away. Why not donate them to Goodwill or DAV - "Disabled American Veterans", etc. ?That is where they would do the most good. And especially since it would disqualify you from Media Mail.( good point Frohike)

Yes, as long as the nonbook item is completely free with the request. I would check the nonmedia rates before offering though.

General Rules in the Book Bazaar:
(3) No selling books for anything but credits, and no offering anything but books for credits. Freebies/incentives can be "non-swappable" items, but remember that non-Media-Mail eligible items CANNOT be sent using Media Mail postal rate. For example, DVDs and VHS tapes and magazines and pamphlets/booklets without ISBNs are all NOT permitted to be posted at PBS; they can all be offered as freebies/incentives, though. But if the freebie/incentive items contain advertising (ie, magazines or booklets bought at a supermarket checkout stand), or are not Media items (anything but books, CDs, DVDs, VHS tapes) they can't be sent Media Mail, so the whole package will have to go by another shipping rate. DVDs/VHS tapes/CDs can be sent using Media Mail postal rate.

I think the non-media rates are going to be quite a bit on some of thsoe things. It might be better to just donate them somewhere.

Freecycle them!

and congrats on your weight loss.