1) You don't want to "replace your soft-modded XB's HDD with the seller's". The seller's HDD is broken. There is absolutely no point in trying to replace your HDD with his.
2) HDD's a locked to motherboards, yes - But DVD drives are not. There's absolutely nothing stopping you from buying this console off eBay, taking the DVD drive out, and sticking it in your current X-Box to create a fully functional system.
In case you are not yet clear on this, a HDD is a "hard disk drive". The console uses this to store data internally (eg, your save games). Unmodded or softmodded consoles require locked drives, hardmodded consoles do not. A locked HDD will only function in the console it is locked to.
A DVD drive is used to read DVD discs. These are fully interchangeable between all original X-Box versions and are not ever locked.
The two are entirely separate devices inside the console.
3) My last few posts offered suggestions for fixing that error 07. My previous post covers ALL your options - if the drive is truly dead, then you either build the EEPROM reader or you install a modchip. I didn't re-mention the two above points re fixing your original console, as they were covered very early on in this thread.
4) Just as an extra point, if you can get both consoles running, you can connect them via a network cable and transfer all files through that. This means that one functional DVD drive would service two systems.