I am wondering if there is a tool available that will allow me to format only a portion of the HDD? For example, despite having 2Tb disks, i know that i never filled anymore than 200gb on any of these drives at any time, so i'd like to run a tool that will say, wipe the first 200gb only?
Even on an empty drive, new data is not necessarily written at the "beginning", it's quite unpredictable. You could open the drive with a hexadecimal editor like WinHex, scroll rapidly to see the areas containing "something" (instead of only 00 of FF bytes), define a block of data encompassing each area (ideally only one large area in the first few hundreds of GB), then fill that block with 00 bytes (with WinHex : "Edit" => "Define block", or right-click => "Beginning of block" / "End of block" ; "Edit" => "Fill block..."). But since zero-filling is an unattended operation, and is not that long for a 2TB drive (for 4TB drives or more it's another story), so unless you've got a dozen HDDs to re-sell, one has to wonder if it's worth the added tediousness for the sake of saving a few hours to get it done. And you don't need multiple passes — after a single pass of zero-filling noone on this forum or even at the CIA would be able to recover a single working file.
Will the first method be safe enough destroy data?
Depends... It's unlikely in this case if those drives were never filled, but for instance, on a Windows NTFS partition, the MFT itself (Master File Table) can contain files in their entirety, if they're small enough to fit inside their own MFT record (1024 bytes). And the MFT is not necessarily in one location, it can get fragmented, especially if a partition was used to store a very large number of small files (I had a kind of reversed situation where I thought I had recovered an area of interest plus the whole MFT on a failing 3TB HDD, but in fact there was a chunk of MFT near the middle, and yet another near the end). Or if at some point the drive had several partitions, there are remnants of data and remnants of MFT records at least at the beginning of each one of the former partitions.
Bottom line, if you do care about safety, a complete zero-fill is necessary and sufficient. If you don't care more so much as to do it properly, don't do it at all, most likely the buyer won't care either and start filling it right away (although, according to Murphy's law, shit tends to hit the fan more often in the vicinity of people not carrying an umbrella).