There is no one size fits all business model, including how postage is done. When eBay says "buyers prefer free postage", what they mean is "we have, through undisclosed (and likely biased) data collection, determined a stat that suggests sales may increase by an average of X% for some sellers when they use a free postage business model".

Stats and percentages are useless without context, the best they can do is provide minor insight into trends, that in turn may or may not work if adopted by an individual. 

This sort of thing also excludes data on alternative business models that can work significantly better for some - eg:

Flat rate postage (one single amount for any number of items being purchased)

Capped postage (postage accrues until it reaches $X amount, and that is the most that will be paid for a single order)

Calculated postage (postage is calculated and charged per individual order, based on the items and postage method required)

All of these models can incorporate a free post option, as well, for example if the total order value reaches a certain dollar amount and / or number of items being purchased.

Which one suits will depend on the items, how buyers tend to purchase (single or multiple items), the actual postage costs that need to be covered, and in some cases, the prices of your items.

In general, I've found that if incorporating postage costs into the item price (to list as "free post") makes the price look high compared to other similar items, charging it separately will usually work better, regardless of whether it is a low or high priced item, but a little more so on lower value items, as people do still place separate values on item prices and postage costs. 

If you'd like to encourage multiple-item purchases, charging postage separately can be one way to do that, if you provide good rates for additional items, so that rather than pay eg $10 + $9 for one book, someone may rather pay $30 + $12 for 3 books. This sort of model is a particularly good strategy when item price and postage costs are of similar amounts (many of my items are under $5, and so is my postage cost, but I use flat rate, which means someone can pay $4 + $3 for one item, or $40 + $3 for ten items, most people will buy at least two items to get better value out of the postage cost). 

Of course, with free post across the board, you can still provide discounts and incentives for multi-item orders via promotions manager, eg a percentage off the total order value when X number of items are purchased, so exploring and experiementing with various options that appeal to you, to test how buyers respond, is really the best way to figure out the best model to use with your items.