Marketplace e-bike buying questions

I don't usually use Marketplace, so figured I'd ask.

Is it strange to ask someone to provide proof of purchase/receipt --OR-- if they can't provide a receipt, the place and date of purchase with s/n on an almost new bike (<10 mi on odometer)? Especially if price is on the low side?

The seller got pretty angry about it. They said their ex bought it and accused me of scamming.

Like WTF?