Is it infringement if I use images from online sources like clip art (et. al.) for creating works for sale?

If perhaps there is a watermark on the cliparts, yes. If the site where you got it says that it is FREE clipart, no, it is not infringement. Just be sure to credit the site where you got the cliparts (if it is free) or ask permission from the site or artist where you got the cliparts first before you use them.