WYB
WYB has 2 meanings
When you buy
In online marketplaces, WYB stands for "when you buy." Sellers use this acronym when detailing deals you can take advantage of WYB an item.
For example, an eBay seller's listings may state "discounted shipping WYB 5 or more items." This means that, if you buy five items from that seller, they'll reduce your shipping price.
Couponers also use WYB to stand for "when you buy," when discussing coupons and other deals in couponing forums. In this case, couponers are likely referring to coupons that feature a WYB deal (such as "buy 4, get 1 free").
Example
Free shipping WYB $50 or more of items
Take the pop culture acronym challenge
Gamers use WYB as shorthand for "watch your back." This acronym may be an entreaty (e.g. "WYB, we can't keep feeding the other team.") or a threat (e.g. "WYB, I'm coming for you."). When playing a multiplayer game, your teammates may also tell you they are going to WYB - which means they'll do their best to protect you from enemies.
You may also encounter WYB on social media, used similarly to how gamers use it. For example, a toxic user or troll may tell you to WYB if you post something they disagree with.