When I list something on eBay, my practice is to make the starting price the lowest price I would be satisfied with getting. In the case of items such as an Encore barrel, in many cases, the selling price will be significantly higher using this strategy than starting with such a high price that no one wants to bid on it. I've seen many used things go on eBay for more than or close to what I could get them for new.
Having watched eBay for Encore barrels for some time (and having bought quite a few), I would say yours has a good chance of going for something over $200 if it is fairly unblemished (this is not what I would pay, however). If I were auctioning it off, I would probably start the bidding around $135 with a $15 shipping charge but your starting price is entirely up to you and you take full responsibility for it. Your best bet is to start your auction in the evening and have it end on Saturday or Sunday evening or better yet when there is a 3 day weekend because of a holiday.
The other auction site I watch is
www.gunbroker.com which, in my opinion, would not be the best place for you to auction this barrel.
Make sure you describe your barrel correctly when you list it. Is it really a straight "bull barrel" or the much more common heavy barrel?