In my experience, if it's online and cheap then it's probably junk. T&G is probably far better than lapped boards, but even so I think I'd want to see some real-life examples first. I saw a review for one recently that offered a ten year anti-rot guarantee. It suggested that was no major risk for the vendor because it would have blown down within the first twelve months anyhow. Roofing felt is often paper thin and the floors made of thin OSB supported on 30mm square battens if you're lucky. The roof can be inadequately supported too and sags under its own weight before too long. It seems common for the entire thing to be held together with cheap staples. Oh, windows are often nasty bits of thin plastic rather than a decent sheet of polycarbonate or anything like that. The doors are rarely weatherproof and with the doors can become even less so because they're made with inadequately dried timber that shrinks badly.
We've bought three since we've lived here, the first to keep beekeeping kit in, the second (bought by my father-in-law) as a garden shed and the third because we needed some temporary storage space for the contents of the cellar when it was being tanked and made habitable. I've replaced the entire roof on all three, including adding bracing so they don't sag. Two have had the floor replaced. When I've had large offcuts of OSB I've screwed them to the insides of the walls to stiffen the structure (which makes a big difference).
If you have the tools and the skills, building one from scratch quite possibly gets you a much better end product. For a 12x8' shed you can probably get away with a 2x2 frame and you can use lapped featheredge boards in that case if you want, because they'll can be put on a lot more carefully and won't warp like crazy within six months. And use EPDM rather than felt to cover the roof.
I moved my bee shed last winter and at the same time as replacing the floor and roof I lined the walls with breathable membrane and added a covered area open on the long side for storage of brood boxes and supers.
James