I always struggle to come up with a user name on any site! I guess it doesn't really matter what you call yourself on ebay - unless you sell lingerie for example it's best not to call yourself skidmark
I have however noticed a connection of some kind between user names and products bought/sold. I have 2 accounts on ebay. My longstanding account is for selling books and have have a book related user name. My other, more recently started, account sells more collectable, country style items and I chose a name to suit. I have noticed that not only do my customers often have a user name that matches the items they are interested in but often have a house name, or even road name that matches! Book buyers on the other hand tend to just have a straighforward user name like harold55 or JJharper. Of course no one searches for items by user name but maybe once landed on an item they are interested in, they , maybe subconciously, are re-assured by a user name. So, just on the offchance that this could be the case, it may be worth thinking of an apt name.
I've made the task more difficult now haven't I?
If you are going to be selling children's clothes and toys etc maybe come up with something that indicates family/children.?
I used my first name as a username years ago and kind of wish I hadn't. It's not a normal name and it's likely people don't realise it's a name, but I still think I was boring/stupid. I'm not good at coming up with usernames either.