SMS wrote:
My sister in-law bought a plastic shed at Costco (Keter Bellevue) with
floor dimensions of 100.5" x 80.5".
("http://outstanding-keter.co.il/files/17185841_a.pdf")

Someone at Home Depot said that she should set it on a pad of concrete
blocks that sit on a base of sand and gravel, all of which she ordered
and had delivered.

I thought this was a pretty bad idea for a plastic floor shed, and that
it will be difficult to get all those blocks arranged into a level,
even, platform. I would have built a base out of pressure treated
plywood sitting on a frame of 2"x6" pressure treated studs, and set that
on level ground without using any gravel or sand.

But now that she's bought all that material, is practical to sit it on
blocks, placed in gravel, even if it weren't an optimal arrangement? I
also thought of putting pressure treated plywood on top of the concrete
blocks and screwing the plywood to the bocks, but that's a lot of screwing.

Not sure (never mind)why she asked the experts at home depot but their
suggestion does sound ridiculous.

You might want to do some checking for accessories. My neighbor bought a
plastic shed like that and the separate base was a set of heavy
interlocking plastic "tiles". They spread some pea gravel and put the
base down on the gravel.