Unless it's one of the few smaller jets approved for single pilot operation, all of them require a two pilot crew, and that means training at least once a year. Then there's insurance, maintenance and hangar costs, not to mention fuel and reserves for engine inspections.
Sure, the initial price to buy an older jet is pretty cheap for some, but it's the daily costs and the unexpected maintenance items that keep it out of reach from most "average" people.
On something like a Westwind or a Hawker, a mid-sized jet with 731's, it burns roughly 300 gallons the first hour and 225 every hour after that. I have no idea what something like a GII burns.
On the Hawker, we just replaced one half of the windscreen because of a heating element failure, costs, about 30K.
Unless you have some reason to use it as a business tool to make money, it's a very large money pit for just a toy.