I got one of those codes as well (I joined in '99). However, is it a fact that the coupons only work on Buy It Nows? I hope not, but I'll find something BIN if that is indeed the case.
I think the restrictions were that it couldn't be used for a best offer accepted. I thought it was also restricted from regular auctions too, but it looks like that worked for Paul Maul.
Here are the restrictions from eBay:
Who's eligible for the Coupon?
Only invited, registered eBay users who see the Coupon in My eBay Messages are eligible. Sorry, no forwarding—Coupon is not transferable.
How to redeem your Coupon:
1. Shop for eligible items. (See below for exclusions).
2. Enter the Coupon code in the redemption code field: HAPPY25
3. Pay for your item by 11:59PM Pacific Time on September 10th, 2020
Terms & Conditions:
This Coupon is a $25 discount off a minimum purchase of $25.01, valid through 11:59PM Pacific Time on September 10th, 2020. Discount applies to the purchase price (excluding shipping, handling, and taxes) of eligible items on eBay.com, cafr.ebay.ca and ebay.ca. Eligible items exclude warranties and protection plans, as well as items from the Coins & Paper Money, Gift Cards & Coupons, Vehicles in eBay Motors, and Real Estate categories. Coupon must be used within a single transaction (and can include multiple eligible items), while supplies last. Max one redemption per user.
Coupon does not apply to auction-style listings or purchases made through the Best Offer feature.Only eBay users registered on ebay.com, ebay.ca and cafr.ebay.ca, with an address located in the United States or Canada are eligible for the Coupon. Any unused difference between the discount amount, as shown on the Coupon, and the purchase price of an item(s) in a single transaction (or cart) will be forfeited.
Coupon is funded by eBay; Coupon is subject to U.S laws, void where prohibited, not redeemable for cash, has no face value, and cannot be combined with any other Coupon, or when paying with PayPal Credit Easy Payments or Gift Cards. eBay may cancel, amend, or revoke the Coupon at any time including in response to fraudulent activity.
If you are attempting to use this Coupon to buy items from both managed payments seller(s) and non-managed payments seller(s), your cart may be divided into two separate transactions. If that happens, this Coupon will be applied to the first transaction if it meets the minimum purchase requirement. If the first transaction does not meet the purchase requirement, this Coupon will not be applied. If your second transaction meets the purchase requirement, you must apply this Coupon during the second transaction.