It looks like eBay may have removed some of the category restrictions on the use of spendable funds. There may be other reasons that you cannot use the spendable funds as defined in the policy.
https://www.ebay.com/help/selling/spendable-funds/spendable-funds?id=5305
Personally I have never used the spendable funds since as a seller there is no benefit in it. The money sits in eBay's coffers and does not draw any interest. I get no discount if I use the funds to buy something even through eBay is avoiding the merchant fees. I am better off charging the items to a cash back credit card and paying off the balance when it is due.
