Add random timing element for user-sold planes appearing on UM

[sanabas]

Selling planes very cheaply to a specific other airline is generally not allowed. It's easy enough to prevent it happening via a private listing, by simply restricting the price range of private listings. But it's very easy to co-ordinate with the other airline to make a public listing that only appears to the public for a few seconds. I change the price of the plane I have for sale to be publicly available and very cheap (because it's been on market unsold for some time), a second later my friend refreshes the UM and claims it.

What if listing a plane on the public UM, or changing from private to public listing, or changing the price, caused plane to disappear off the UM for up to 2-3 hours, and to only appear on the UM in one of the next few periodic market refreshes? Our airline staff take 2 weeks/7 RL hours to organise delivery when we buy a plane from the UM, so we could say they take a few days/up to 3 RL hours to organise getting the plane listed and ready to sell.

That'd make it significantly harder to coordinate public sales. If you're putting a plane up as a public listing, then it shouldn't matter to you who buys it. This way, other airlines have some chance of seeing the bargain first and claiming it, same as they do for claiming broker-listed planes. If you're trying to sell to a specific airline, then you just list it privately, and adhere to the narrow allowed price range.