The prices that you are willing to pay for a coin will eventually set the guide price for that item in the future. Ebay is a database & companies pay for historic data  provided by companies such as Terapeak to prove the market trends. If you are buying or selling graded coins below the trend price the end result is the data will drive the market price down. If a buyer was to bid higher than the trend price, then the data will be used to reset the market value at a higher price. With todays technology, what you are willing to pay for a coin today will set the trend price next month.