i'll say go with a new coin because that way you will have a stand alone currency that doesn't rely on anything else. for example if you create a token and something happens to that platform (like the congestion you said) you can't do anything about it and you have to bear the consequences. but if it is your own coin you can come up with solutions.

but at the same time i don't see any reason why you do either of these two because of your purpose! you say it is for item trading. so if you restrict people to a certain coin/token you are restricting your audience too. and most people regard altcoins purely as an investment to play around not something they actually use. which will also restrict your customer even more.