good intention, bad idea imho.
people will go in there to find cheap / free bits just on the off chance they will use them rather than those who could do with the part & are either financially embarrassed or similar.
maybe have a karma shake-up in the fs section where all karma items have to be put into their own threads, marked as karma & a time for the lucky number to be drawn is mentioned in the post? this has worked well, and very fairly for larger / more expensive items so maybe encourage the use of this style of giveaway rather than first dibs.