We have a resin shower tray ( not sure if its " slate effect" ), and I hate it. Looked lovely when it was installed. But it began to develop small chips in the surface fairly soon after installation . I spoke to the Retailer who told me there were repair kits available to repair the chips ( which worryingly, suggests this is a common problem.) I dug my heels in and eventually they arranged for somebody to repair the chips. Further chips have developed since.

It is impossible to clean it properly, the care instructions advise that cleaning with water should be sufficient, it isn't. I've tried everything to clean it and it is just not possible. I think the uneven surface doesn't help, it seems to trap dirt.

Given a choice I would go back to a tiled floor in the shower but that would be a major upheaval now.

The tray you are suggesting may be a different product but do do your homework.

I've just looked at the Tissino slate effect shower tray and it looks very much like mine. (Mine is not Tessino) Ask the Sales Staff if they know of any reports of the surface developing chips.

We chose a quadrant enclosure because our bathroom is small, and it's been the right decision. The slate-effect tray still looks good, although hard water means you'll need to stay on top of limescale. Our rimless toilet is definitely easier to clean than the old one. We also used porcelain tiles, and they've been the most durable and low-maintenance part of the bathroom.

Terrazzo Pastle Porcelain Tile · More Info

The OP is a bathroom seller!