Speaking as a "retired" army builder (who still builds squads of 5 of troop/army type figures), I can say that I'll readily have extra droid parts. Not all of them to complete a whole droid, but some of them. I'll most certainly trade parts or outright sell them to people who want them who don't have anything in trade. Cheap of course. I'm not out to make a profit.
Tentatively I'll say $1.50 for each part plus shipping. Definitely not more. Maybe less depending on the price of figures in my area.
I'm not taking orders for these, so please no one call dibs until I know which parts I'll have extras of. I can say I'll have several extras from the Legacy Clone Wars wave as I want multiples of the Quarren, Mon Cal and Scuba Trooper. So whatever parts they contain, I'll at least have a few extras of. (Again no calling dibs though until I get those parts and post that I have them for trade/sale.)
People for whom I've traded with in the past will receive preferential treatment. Sorry, but it's only fair to them, since I've received good deals from them. One hand should wash the other, so to speak. Though it won't hurt to ask me even if you've never dealt with me before. Who knows how this whole build-a-droid thing is going to succeed or not.