I would say no to both.
1) Who's to say your buying a used XBL membership or counterfeit? I found XBL Gold membership cards on newegg.com and Amazon for $29.99 - $39.99 when they are on sale. Some new games come with a free trail of XBL Gold.
2) As for the battery who knows if it's been used. If so it's not worth it. You can buy a rechargeable battery for like $11.99 at Walmart.
My advice is to check the sellers history if it's in the upper 99% then it might be worth taking a risk if you feel comfortable. I also advise shopping around by going to places like Amazon.com, Fatwallet.com, Newegg.com and others. You'll be amazed at the specials that go on. Dealrush.com and Cheapassgamer.com are two other sources for deals. In fact Dealrush.com post their daily internet special from various vendors.