Questions : Selling here for the first time

If you are in the US, only sell to a US location at first. It simplifies things greatly. I typically ship FedEx Home Ground. It is usually the least expensive. It may be a bit less expensive if you have a FedEx Account. Insure for the full sales price and require an adult signature.

Always over package your items. Sometime even the original factory inner packaging material is inadequate. Add more packaging if needed and double-box when possible.

It’s definitely more convenient, less risk, and less cost to ship within the continental USA. I generally sell USA only, but sometimes certain big-ticket items (e.g extremely un-obtainum OOP high-end headphones) are much more highly in demand in Asia/China, and that makes it tempting.

That said, once you get packages over a certain size and weight (and you will often end up with big packages if you’re packing properly), it can be surprising how much it costs to ship ground from one US coast to the other.

And prepare yourself for an absurd onslaught of "why are you selling?" questions, lolz. A couple times now I’ve gotten tired of the tire kickers, gave up on the ad, and later became thankful I never sold that piece (i.e. further tweaks, additional burn-in, changed system balance, etc)! When I want to buy something, I know I want it -- so I offer a generous/fair price right off the bat (often asking $) and avoid the BS -- purchases go very smoothly this way. But a LOT of buyers here want to feel like they "bargained down", regardless of the start/end price, so you might consider padding your initial ask price to account for this. Stupid habits, if you ask me. 

Shop around for best shipping price. Pretty sure UPS stores are independent so the owner may try to gouge. I've walked after given a price and saved $30 at another location (FedEx).
Be very specific (and truthful) when writing your ad. If anything is wrong with your item (scratches, dents, etc.) say so. If you have a "no returns" policy, say so and try to get the buyer to inspect and accept ASAP (and get it in writing).