I'd wager that the correlation between "likelihood of scamming" and "time between join date and donation date" is stronger than it is for "donor amount in dollars" (percentage-wise, at least). Pretty sure we've had scammers up through the $1k donation level in the past, and I'm sure I'm forgetting some.
What could be interesting is a suggestion asking for more information on users' donation timing, in addition to donation amount. For example, knowing the difference between someone who donated $500 six days ago vs. someone else who donated $500 six years ago could be valuable when trying to decide who to do business with in the marketplace. There's definitely some aspect of this which users would want to keep private, and I'm not advocating for full disclosure on this stuff, but I'm sure there's more we could do without too much trouble.