+1 Search ebay history. Since you are experienced on ebay, you are already aware of the tremendous variance on ebay. The same item, at the same time, can sell for 1X, 2X, 1.5X and .5X. I see it all of the time. The bidding is very uneven. So I would not count on getting a number.
+1 If you know nothing about mechanics, I would skip the entire process. Keep the bike with its original parts, or buy a bike with the SRAM parts you want. Paying someone to do the work (both the take off and reassemble) will not be cheap. Its best suited for someone with the time/tools/aptitude and interest in wrenching on bikes.
As far as a difference between new and buying take offs on ebay, which would you rather buy, and where would you pay more? Would you pay some unknown person on ebay, who claims the parts are brand new, or would you rather buy brand new stuff from a local shop, or via the internet? I think the answer is pretty obvious.
Brifters are an area on ebay that is really buyer beware. It would be one of the last parts I would buy on ebay. It would have to be a smokin hot deal for me to consider them. Full disclosure: I bought a mini-group of nine speed Ultegra last year on ebay, complete with derailleurs and brake calipers, handlebar, stem, both brifters, and cables. But the seller really botched the listing, and I paid about what the two derailleurs used would sell for (buy it now auction). Turned out the front brifter was a little balky, but I got it working. And at the price, it was a great deal regardless.
Right now, there are some really good deals on brand new drive train groups on the internet (European sellers). The drop in the Euro and the pound have made some of the deals compelling: Probike Kit, chainreactioncycles, etc.