When you shop out of season it is important to realize that you're shopping at the time those items you wish to list are in clearance or were a few months earlier. What I do is I make sure that I buy everything that I find in bulk in clearance, meaning if there are five tablecloths one table cloth has two or three and the rest have five or ten the one that has two or three I'll buy it. I listed immediately. The one with five or ten I'll stick in a box and I'll list in 30 days. The smaller item is my tester pre-season.
It is important to look at HUB on eBay or whatever platform you're using to determine what sales levels are on products at the current time. I check my records from the previous year. I look at the current demand traffic all of my items and information from the last year and determined is that trend going to continue product lines that I know will be popular year after year. I determined based on price availability and product demand how many new products to purchase in clearance or during the off season. When a new product line comes out in season I also determine is the new product more attractive or the older product.
If I have the opportunity to buy the older product I do. The new product I buy some at the same time quite often. If I think the old type is better I will Overlook the new product and by only the old product. It might have been more popular also Demand on places like eBay for items that are no longer available in retail stores is a huge area of eBay business or at least for me selling the items I carry.
In other words do things based on availability and market demand online.
It is important to pay attention. Attention to details such as unit cost to purchase and availability of units is as important as demand at the current time.
If you list every day to keep your store "bots" crawling and buy items at low cost of value and show that value to your buyers you will succeed!