Definitely find ways to use these miscellaneous items, even if it is just to fill out existing baskets. If the items just don't go, you could put together a basket of "odds and ends" and do it as door prize, or $0.50 grab/raffle.
TeaPot100 - I would never turn away donations, even if they are unrelated to the specific theme of an auction basket. You can do a few things with the extra items:
1) Create a new basket to be used in the auction
2) Create a small basket to be given out the person that spends/donates the most money at your auction
3) Use the items for the first 10 people who check out first. Basically a small incentive to get to check-out fast.
4) Paste/stick them under chairs and have everyone look right before dinner or the auction to great excitement.
What do you do with donations that are unrelated to the specific theme of an auction basket? Do you say 'no thanks'? Maybe put it in a grab bag basket, shroud it in mystery with other unrelated items? lol.
Also, what if on a sign-up genius, let's say you provide a list of items to donate with the intention that people will choose 1 or 2 or all of the items and 4 more people will know not to donate duplicate items and you even say 'indicate what you are donating so there are no duplications'. But let's say people get confused as people will do and so 4 other people sign up to donate the same thing. How do you handle that? Accept those donations anyway and have a basket that has 5 hand lotions, 4 deodorants and 1 toothbrush?