If you're shipping to the US, try a company like Chit Chats (they have a location in Montreal but I don't know how close to you). They will ship through USPS for you and it'll be significantly cheaper than Canada Post. They should have a price chart that will give you a good idea on how much something will cost to ship.
Get a Canada Post Small Business card on their website. It doesn't help much but it doesn't hurt to have it. You'll be able to send expedited parcels for the same cost as regular parcels.
If you're buying shipping through eBay, it'll make your life easier to have packing materials and a scale on hand. I bought this one and have no issues with it. The time it will save you at the post office makes it more than worth it.
I haven't bothered with a PO box. I've considered it but decided it wasn't worth the cost and haven't had any issues yet.
Use PayPal. You will always hear the horror stories, no one posts about PayPal unless they're having issues.
You can get large envelopes at the dollar store but if you're going to be doing this alot, look for shipping supplies in bulk. eBay, Amazon, Uline, Staples, etc, all have some form of materials. Do some shopping around for the lowest rates. Just don't buy the supplies from Canada Post (probably the most expensive).
If you are going to use Canada Post, just get the dimensions and weight filled out on your eBay listing and use calculated shipping. As long as your measurements are accurate, the buyer will be charged the proper postage cost.
I've been happy with this one: https://www.amazon.com/dp/B00I9D5IFM?ref=ppx_pop_mob_ap_share Almost every scale on Amazon should work fine, as long as the weight capacity is high enough. There is one feature I want to point out, though! Once you get over 5lbs or so, it's helpful to get the style where the display screen is separate from where you set the package on the scale (it's a lot easier to read!)
If you want to do a closed transfer a scale is the best way to know for sure.
This is what I use. https://www.amazon.com/Smart-Weigh-Digital-Shipping-Postal/dp/B00I9D5IFM/ref=sr_1_2_sspa?ie=UTF8&qid=1506086964&sr=8-2-spons&keywords=postal+scale&psc=1
I also use it to accurately blend sour beer by weight.
I actually use a digital postage scale. Might be worth a look.
>https://www.amazon.com/dp/B00I9D5IFM?ref=ppx\_pop\_mob\_ap\_share
I've purchased a couple of these over the years. They work great!
Full disclosure.. The first one broke after about a year and a half of heavy use.
I use this...
https://www.amazon.com/gp/product/B00I9D5IFM/
The controls and display are attached with a cord, so large packages don't block them.