Any member can sell at any time.
Those of us with Stores are usually selling large numbers of items (over 50) and using Fixed Price listings, although Stores may also have auctions.
The advantages are with fees and with labour.
Listing at Fixed Price /Good Until Cancelled usually costs 50 cents a month for a casual seller. For a Store Owner, this drops to 20 or 5 or 3 cents a month, plus monthly "rent". A considerable saving.
Labour is saved because the GTC listings will automatically relist without any further work. Over my seven IDs (yes, you can have multiples) I have around 10,000 listings. I really don't want to manually relist each month!
Disadvantages are that Store owners are not eligible for the Fifty Free Auctions promotion and are rarely included in other promotions.
For a new seller, going with the 50 Free Auctions is the cheapest move. Did you know that you can get another 50 Free by listing on eBaydotCOM? There are shipping questions there of course.
And you know already about the 21 Day Hold on your buyers' payments right?