Hi Alan,
I'm a NOTHs seller, but it was a mistake. It works best for designer/makers who don't have the profile to have a successful website of their own. Before you sign up, they'll lay it on very thick with the flattery and constantly calling etc. You then pay a fee to get in, and 25% commission +VAT on everything you sell. The joining fee is similar to taking out a print ad in the back of a national magazine, which was what swung it for me - it has a similar level of impact, but lasts longer.
The rules go on an on. You aren't allowed to charge more on NOTHs than you do on your own site. You won't be allowed to sell on other marketplaces. You have no access to customer emails for marketing, and must not include anything with your brand on in the order packaging. They will buy AdWords for your business name. You have to pay a processing fee on refunds. If you have your own store, it's a struggle to see how you benefit at all. Unless you're making your own things and have a massive mark up, it's just not worth it. To be a successful seller there you're more like a franchisee than a business owner, you're certainly not master of your own destiny. Over time I have decided that I'd far rather give my 'proper' customers 25% off than sell stuff via NOTHs, but it's not necessary. Stuff shifts at full price on my website anyway. I haven't closed the storefront, but do not push it at all.
Yes they do a lot of marketing, but no one gets in the catalogue without paying. A lot. You'll notice in the catalogue that the products are mentioned by reference number only, so even though you've paid to have your product featured, it does nothing to raise awareness of your brand. If you go in the catalogue, NOTHs will give you a sales prediction. A lot of people have been stung buying in stock & materials to meet the prediction which has turned out to be rubbish and they're out of pocket and stuck with stuff they can't shift. All of the products featured on NOTHs front page or mailouts are ones from sellers who consistently pay to be in the catalogue, which is apparently a complete coincidence.
There's a NOTHs seller forum which is very similar to the eBay seller one - full of people moaning about how it's all gone to the dogs and how they're being ripped off. The site has grown massively in the past few years; there is a lot of very similar stuff on there and frankly, you'd be better off spending the money on AdWords or something.