Are you sure it only takes 50 characters? I’ve just checked mine and I’ve apparently got 413 characters. Anyway any MS office word or equivalent should give you a character count. I use a PC with libre office writer and at the bottom of the screen there is a page count followed by a word and character count, the other half runs a mac with macword 2008 and if you go to tools there is a word count function which also gives a character count (might need to highlight the text first).
Also you have both the buying from me box and a meet the maker box at your disposal. The buying from me box just needs to have details of postage services used, returns policy etc. The meet the maker box can have more info like your biog, inspirations and whether items are one offs, limited editions etc so you can spread out the info so that you don’t max out the character count.
Hopefully that helps