The delivery question for artwork is really a framing question, and it should be answered in the listing.
Unframed
Works on paper go flat between rigid boards in a flat box, or stay rolled in a wide rigid tube if they are already rolled. Stretched canvases go corner-protected and double-boxed. All of that posts fine. Packing art.
Framed
Glass in a parcel is a risk to the thing it is protecting. Either take the work out and post it flat, or offer collection — and say in the listing which you are doing, so a buyer can choose before bidding rather than discover after.
The four options
Courier for unframed work, properly boxed. Your own delivery locally, which many artists offer and buyers like. Pickup for large, framed or valuable pieces. Buyer arranges for very large work.
What decides it
Size first, then whether it is glazed. A large canvas is a volume problem rather than a weight one, and most couriers price on both. Measure the packed box before you quote; an underquoted parcel comes off your own balance. How delivery is charged.
When collection is the honest answer
For anything over about a metre, anything framed under glass, and anything irreplaceable. Pickup terms are agreed before bidding with a collect-by window. How pickup works.
Combining wins
Works on paper combine well in one flat box. Canvases do not. Set what each later win adds with the larger piece in mind.
Questions people ask
Should I sell framed or unframed?
Unframed usually sells better and travels better, and the buyer chooses the frame.
Can I roll a stretched canvas?
No. Only unstretched, loosely, painted side out.
What about my own delivery for a local show?
It is one of the best things a working artist can offer.
More: setting up delivery, selling art.