I, and many others, get adds all the time from people adding me to buy/sell me something based on month/s old thread/s. How do these people find the trades? They search google, and they find a trade with really good prices (as there months out of date). So if all inactive trades get moved to a thread that google can't see (or doesn't search), then necro-buyers will be a thing of the past.

This ends up serving two purposes.

1) It stops necro-buyers from seeing old trades (which are clearly invalid, i.e. trades selling keys for $1.30 USD)

2) It keeps fresh/active trades easier to find via a google search.