What is it that determines if an item listed on ebay, sold or unsold, disappears after say one or 2 weeks? (I have found that items disappear after about 6 months but these are more recentthan that, wihtin last month.)

For example, I bid on a bike (pedal) which did not reach reserve and now it no longer appears in my "not won" list nor can I find any trace of it in completed items.

Another bike (motor) <--- linky
was in a no reserve auction and top bid was about 8 K. Now it is up for £ 18; original seller was in Worcester so I am wondering if a) it is same seller who realised that a no reserve auction is gonna take the p155 and decided not to sell or b) it is the buyer trying to make a huge profit.

The item does not appear in completed items so back to original question - can a seller delete old items or how else do they disappear?

I think they stay there for about 90 days then they are purged to save disk space on ebay's pc cos they can't afford a bigger hard drive

I agree about 90 days purge but these items weree definitely within the last month, if not within the last 2 weeks.