Published: 30 May 2026 by Dan Williams
Series: Notes on eBay & Discovery
Some of my observations on why related items sometimes appear to sell together.
One phrase I have heard used recently is “Green Wave”.
This term appears to have been popularised by eBay reseller and YouTube creator Polish Pete (YouTube channel: @polish_pete_uk). He uses it to describe a phenomenon some eBay sellers will recognise: sometimes, when one item sells, then several similar items seem to sell shortly afterwards.
I first came across the phrase through his videos and found the observation interesting enough to start paying closer attention to my own sales patterns.
The idea is simple.
You might go days or weeks without selling a particular type of item. Then one sells. A day or so later another sells. Then another. Sometimes several appear to move together in a surprisingly short period of time.
The interesting question is not whether this happens.
The interesting question is why.
The Observation
I’ve noticed similar patterns while selling stamps on eBay.
Not always and not predictably, but often enough to be interesting.
For example, I may list several related Machin varieties, watermark varieties, or stamps from a particular reign. One item sells and, over the following days, similar material appears to move as well.
Sometimes these are newly listed items.
Occasionally they are items that have sat quietly for months.
The sales are rarely identical. They do not all happen at once. Yet they sometimes feel connected.
Whether that connection is real or merely coincidence is difficult to prove, and an impossibility for me.
However, the pattern appears often enough that I find it hard to ignore.
One Possible Explanation
Polish Pete’s explanation seems to focus largely on item similarity.
His view, simplified, is that repeatedly using similar keywords and item specifics helps eBay understand which items belong together. When one sells, related items become more visible and additional sales follow.
There may well be something to that.
Marketplaces need ways to understand relationships between items; titles, item specifics and categories almost certainly contribute.
However, after reading a number of eBay’s engineering articles, I suspect the explanation may be broader than keywords alone.
Beyond Keywords
One thing that stands out from eBay’s public engineering articles is the importance of relationships.
Modern recommendation systems appear to look at much more than text.
Potential signals include:
- item titles
- images
- categories
- attributes
- watchers
- clicks
- purchases
- buyer behaviour
and likely many other factors.
The exact details are not made public, and I wouldn’t expect them to be.
But the overall direction seems clear.
Marketplaces increasingly attempt to understand which items are related and which buyers are likely to find them relevant.
That means a sale may represent more than a completed transaction.
It may also become a new piece of behavioural information.
The Role of the Successful Buyer Session
One idea I’ve been returning to is that marketplaces may care less about individual listings than successful buyer sessions.
A buyer arrives.
They browse.
They compare.
They view multiple items.
They add something to their basket.
Perhaps they buy more than one item.
That is a successful outcome.
If recommendation systems are trying to maximise successful buyer sessions, then it makes sense that related inventory would sometimes receive additional exposure after positive buyer interactions.
Not because the marketplace is rewarding the seller.
Simply because it is trying to help the next buyer complete a similarly successful journey.
Old Inventory and New Inventory
One aspect of Green Waves I find interesting is their probable effect on older stock.
Many sellers have experienced items sitting unsold for months, sometimes years.
Then seemingly out of nowhere, they begin to move.
I’ve seen examples where some of my newly listed material appears to coincide with sales of older related items.
Again, this could be coincidence.
But it is also possible that fresh inventory helps strengthen the visibility of a wider cluster of related listings.
If marketplaces are increasingly thinking in terms of relationships rather than individual products, this behaviour would not be all that surprising.
Collectors May Amplify The Effect
I also suspect collector behaviour plays a role.
Collectors rarely behave like commodity buyers.
They browse.
They compare.
They revisit.
They often purchase several related items rather than a single item.
That creates richer behavioural signals than a simple search-and-buy transaction.
A collector who buys one watermark variety today may well be interested in another tomorrow.
Or later the same evening.
Recommendation systems do not need to understand collectable items to notice that pattern.
They only need to notice that the pattern exists.
A Better Question
When sellers discuss Green Waves, the conversation seems to be:
How do I trigger one?
Personally, I think a more interesting question is:
What exactly am I observing?
Because the phenomenon itself appears real enough.
The challenge is understanding which parts are caused by:
- buyer behaviour
- recommendation systems
- inventory structure
- similarity signals
- coincidence
and which parts are not.
The answer is probably not a single thing.
More likely it is a combination of several systems interacting at once.
Closing Thoughts
I don’t know whether Green Waves are an official feature, an unintended consequence, or simply a symptom of larger recommendation systems operating behind the scenes.
What I do notice is that related items often seem to move together.
The phrase may be informal, but the observation is interesting.
The more I pay attention to buyer behaviour, recommendations and thematic inventory, the less I find myself looking at listings in isolation.
Instead, I’m increasingly look at relationships between listings.
And perhaps that is the real lesson.
The Green Wave may not be about individual sales at all.
It may be about understanding how products, buyers and recommendations connect together.
— Dan Williams
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This note forms part of Notes on eBay & Discovery, a collection of observations about visibility, recommendations and buyer behaviour on modern marketplaces.