Well, thanks for the contribution, Vikram, and the more receptive attitude. Welcome.
I do think one thing stands out from your post - the issue of pitch. It is NOT normal to consider a note named D or, thence, a flute named as a "D Flute" to actually be an Eb! (This is a separate issue from that of "transposing instruments", where the 6-finger note is called "D" whatever actual pitch it plays.) As I think I wrote before, I strongly suspect your original model was a High Pitch flute - a Boosey Pratten style 8-key flute tuned at A=455 would closely approximate to an Eb flute at A=440. You need to re-scale your product and get it playing in true D at 440 to make a seriously necessary improvement - and to stop misleadingly advertising your current model as "in D" when it is not, so far as modern western players at International Concert Pitch are concerned (i.e. virtually all of us!). I disagree that a simple Google search will enable a beginner with no experience in the field to find out about and understand this. When they see a flute in a music shop or on eBay they will not know what questions to ask and will take things at face value - and probably buy a product that is not actually suitable to their wishes, however playable in its own right unaccompanied. Their ensuing dissatisfaction cannot be good for your business!
I see little point in merely improving the quality of an Eb/HP flute. Produce a flute that truly is in D at 440 and with properly made keys and a decently cut embouchure (and no burnt rubber smell!!!) and you'll get much better sales and a massively better reputation.
I wrote to the current lister of a similar flute on ebay, and he wrote back to explain about the Eb foot. They claim to be copying a modern Irish flute. I think the Eb key on the flute is similar to the same key on the silver Boehm flute. With six finger holes covered the Eb key on the foot is closed, and the flute plays low D. With the same six fingers covered and the Eb key depressed with the pinky to open the hole on the foot, the flute plays Eb, thus a low D flute with an Eb foot.