I think the only purpose of most of those characters is to allow people to get creative with their ebay feedback and make lots of pretty sqiggles on the page.
Actually I think the character ~ is called a TILDE and there is some info about its history on wiki:
http://en.wikipedia.org/wiki/Tilde. They say that "It was originally written over a letter as a mark of abbreviation, but has since acquired a number of other uses as a diacritic mark or a character in its own right, and there are a number of Unicode characters for these different roles. In the latter capacity (especially in lexicography), the tilde or swung dash (⁓
is used in dictionaries to indicate the omission of the entry word" which is essentially wot Emma said.
"Is it ever correct to use æ instead of ae?"
Yes! It is correctly used in words like archæology and mediæval, and in lots of borrowings from Latin, Greek (although of course you can often substitute ae or just e correctly, depending on the word). It is also important in Danish and Norwegian spelling and is a specific symbol in the phonetic alphabet.
<Added>wiki didn't have a smiley in their entry in case you were wondering! Apparently on writewords a tilde followed by a closing bracket equals a wink!