My only concern would be that a scammer took over the seller's account. I didn't notice any red flags tho in my brief look at the auction. The only minor concern about a scammer taking over is that she hasn't had any action in quite some time. So, it does fit the profile of a nice feedback but dormant account that the scammers like to target. However, since there are SOOOO many of these types of accounts, it doesn't necessarily mean a scammer took over.
Otherwise, just watch out for how much shipping is. As for the final price, it may be ok to pay a premium if it really is hard to find this item in stores. The Playstation 3 and TMX Elmo are prime examples.
The Sidekick 3 really only is useful on T-Mobile. So, make sure you're willing to use the T-Mobile network.
What area of the country are you in? Asian cell phone stores will typically give you money if you sign a contract but don't take a phone. Also, GSM phones are easily unlocked. This allows you to use any GSM phone on any of the GSM networks (right now, I think it's just down to Cingular (which bought AT&T) vs. T-Mobile (which uses Cingular towers in some areas). While the Sidekick 3 can be unlocked, many of its cooler features can't be used on Cingular's network. This makes it one of the rare cases where unlocking won't do you much good.
Oh, I forgot about sniping. This is prolly a prime target for sniping. Sniping is when someone bids at the last second. There are computer programs that do this for you. I typically bid 8 seconds before an auction ends. I have two screens open to the item. One is all ready to bid at the click of a button. The other is used to keep refreshing to keep track of the item. The lowest most computer programs have automatically set as their "bid" time is 10 seconds. However, I have been beat even when bidding at 8 seconds.
If I'm really interested in an item, I never bid on it except early in the auction. Bidding only raises the price and lets snipers know you're interested in the item. Since this is your first time on eBay and it's relatively early in the process, it might be a good idea to bid a tiny amount. Just so you see the entire process of bidding.