Similar to the last ‘PROMOSMALL’ there doesn’t seem to be a list of sellers so the applicable sales will have to be hunted.
I’ve found it applies here
How to redeem your coupon:
Simply make a purchase of £10 or more on eBay.co.uk on an Eligible Item and pay with PayPal, credit card, or debit card. An “Eligible Item” will include a reference to coupon code PROMOSMALL on its view item page, just below the box showing the price and buying options.
Enter coupon code PROMOSMALL at checkout when prompted. The maximum discount you may receive is £50 per redemption and you are limited to fifteen redemptions (up to a maximum of 10 items per redemption).
Terms can be found here