Here's the bottom line for ebay: T23s go for incredible range, from $450 up to $700.
I always find out what the low end of the going rate is when bidding on stuff on ebay. Do it like this: run your search, tighten it up to get rid of junk, then check the box to look for completed items. Sort by low price first. Going up in price, look for the spot where there start to be a fair number of winning bids.
The problem with ebay for T23s (or T22s) is that there can be a tremendous variation in the quality of the thinkpads being auctioned. Most of the stuff will be off-lease, and some of it will be pristine and some can be pretty beat up. All you can do is read the descriptions, check the feedback, email the sellers and pin them down on the condition. I got a T22 that was close to perfect -- but the seller had super feedback and had glowng feedback for a T22 auctioned the week before, so I felt pretty comfortable going for it.
As far as wireless goes, very few of the T23s on ebay currently have integrated wifi (I think only 2 or 3 out of the ones that are listed) (I had a little buyers remorse and was surfing to see what T23s were out there

). Be alert for auctions that mention wifi but are actually referring to a PC card the seller is throwing in. Somewhere on this forum are the model numbers of the T23 that are wifi enabled, but I don't have that information any more.
Bottom line is that if you want a wifi enabled T23 off ebay be prepared to be patient unless you want to throw money at it. But compare with IBM refurbs (which you can count on being pristine) for price purposes.
One other thing, most of the T23s being auctioned have 512MB, I'd pass on a 256MB model. Also, few 1.13 Gzh, but the difference in CPU speed in the available range isn't worth worrying about, imo