why do sellers change asking price to sold after a sale?

[G1911]

1) the listed price is often not what it actually sold at. Often it traded. Often I took the OBO option.

2) the sales data for 99% of cards sold here on the BST is easily found by clicking eBay’s sold results to get a much larger data set. Unique stuff isn’t sold here all that often. I just don’t see any meaningful purpose.

3) because some people don’t really read and will continue to send emails even after I’ve said it’s sold or traded or gone, but if “$SOLD” is written over the price they can’t miss it because that’s the part people read.