kijiji... eb games and local game shops are basically the only place that take used systems. but the problem here is that yours is the 1st gen model and won't command a large return on value. eb games will most likely give you way less than the xbox one s model.... same for game shops as they will have it sitting around for a while unless they bundle some older games (sports, or old bargain bin titles) just to move it.
your best bet is kijiji as you can squeeze a bit more out of considering you aren't charging tax, but don't go nuts because if you get close to the tax in value a buyer would at that point just buy from a game shop to have a warranty of some sort. another option is to ebay it... bundle some games with it and any accessories you don't use and ask 5% less than the lowest one you see going bundled the same way. a buyer on there will bite since they aren't paying taxes, they have purchase protection and it's being delivered right to their door... so the value is worth it to them at that point.
i know eb games has that once in a blue moon trade your old system in to get an XBO-X, so you could just hold onto it till that rolls around again and hope they haven't removed the 1st gen model from systems they will accept. good luck !