Yes it happens, although I can't say how frequently. When you have monitored thousands of sales for price research as I have you start to spot these events. Some people also have multiple eBay accounts so they would put something up for sale under one account and then use one of their other accounts to put up the second (less expensive) item. Other people maintain accounts just for buying (never selling) so you can't spot what prices they buy things for which they then turn around and sell a month later. Except that I spot them sometimes because the item may have distinguishing features which I pick up in my tracking.
I've spotted certain sellers who may have a drum or cymbal marked "sold" which later goes back up for sale under one of their other accounts. In the past two years something has gone funny with the eBay outcome codes so you can't even rely on something which says "sold" as actually having sold. But in some cases this may be the seller trying to push the price envelope by appearing to make a sale at a higher than expected price. I've reported such behavior to eBay but I no longer bother because they have never acknowledged my reports and apparently never done any investigation. Multiple accounts are allowed under eBay rules, although you are not allowed to use "shill bidding" which is where you bid on your own item using a second account in order to drive up the price.
All of this sort of thing is why you should only use completed sales for estimated value. In some cases where an item sells for a "silly high" price based on my previous recorded sales of similar items I flag the item in my database to recheck for feedback before I believe that it did actually sell at that price.
Most of my tracking work is still on eBay so that's the system I'm most familiar with. But it is a fair bet that similar things happen on other platforms...and as a statistician I don't usually bet.
