My favorite resale shop in town is Kids Junction (NW Military Hwy). It's quite the drive for me since we're by Sea World, but well worth it IMO.
Just the other day I went to a somewhat new shop near Helotes, called Rose Cottage. They didn't have much bigger baby gear items but they have TONS of clothes & toys. Very nice stuff & for the most part, reasonable prices. The store is super organized, too.
Kid to Kid is "okay", but I don't go to those all that often because they tend to be snotty & downright rude. A little customer service goes a long way & they don't seem to have any as far as I've been able to tell.
Craigslist is good, as is eBay (just search by "nearest first").
There's also the local Yahoo groups (SA GarageSale, SA Recycle, etc.).
Good luck with your searching!
ETA: Regarding the playpen, you might want to check out Baby Depot ( @ Burlington Coat Factory). When my youngest was little I got his playpen from there (NEW) for less than I was finding used ones for. It was on sale & I had a $10 off coupon so it worked out to be a better deal (Graco Pack 'n Play w/ Bassinet -- ended up costing $40 if I remember correctly). Just something else to keep in mind.