This is one of those things where one size is not going to fit all. You could try the way DAZ did it for some time, giving the ‘base’ away and charging for extras. This might work well for your building and environment type sets, e.g. give away one or two parts and sell the rest as a pack or if it makes sense, individual items.

If you go with a ‘donate’ button, think about making it a set donation, e.g. $1, £1 or whatever. Removing the decision on how much to donate might yield more spontaneous donations.

I don’t know if anyone has ever done this but listing how much the item ‘would’ cost if you were selling it might help educate people, at least as long as the felt the price was reasonable. It might encourage a few more donations, who knows. In terms of sales, maybe people could do ‘tie-ins’, e.g. if A is selling a serving wench outfit and B has a medieval tavern, mention the items on each respective page and offer a discount to those who purchase both. It would be nice if the store could handle that automatically.

"Democracy is a pathetic belief in the collective wisdom of individual ignorance." - H. L. Mencken