I think it depends. The kind of situation you are describing certainly needs to be declared to the JSA department. Not doing that is likely to mean you may be breaking the law.
You say it is not a business, but makes good enough money to live on properly whilst not working. Many would disagree that £400 meets your description, good enough for a normal, proper living. But it is significantly more than the basic food, heat, shelter allowance of older JSA recipients, and nearly twice that allowance of the youngest recipients. (But really that benefit is not meant as anything like a "proper" living - it's just to keep people from actually starving and being on the street, having the cheapest clothes in order that they can find a job. You can't really get a job if you are actually starving, homeless, and in a totally terrible state.)
A starting point is - if you are selling your own possessions which you had bought from your own money, these are counted anyway in the same kind of terms as your savings. (There is a limit to the amount of savings you can have and still be in a position where you are thought to need benefits.) If you had these items before you received benefit, they are counted in your savings. You are allowed these savings and you are allowed to convert savings in the form of items into savings in the form of cash. You are then allowed to spend from your savings. As long as you still have LESS than the amount of savings the state says you are allowed to have and still be eligible for JSA (check the gov.uk benefits information web pages for the current amount).
So that only applies to items you already had as your own, normal possessions. Most small, invaluable things you already own will not have to be declared in your savings, but if you have a lot of them, adding up to significant value, they may need to be declared.
If you acquired the items from benefit payments, your own possessions, later sold, it's fine to sell these up to a certain point. It depends on the type of practise.
But when you start talking about nearly £90 pw, regularly, every week, this will change. Obviously, after a while it's unlikely you are selling your "savings items". And, also you wouldn't be considered to be buying some of the things you are selling with the benefit money, anymore, you'll be buying them with income from the sales themselves.
You say it's not a business, but it really is. OK, you're not registered as a company. But what you described, after you've sold off your personal items you already had (that's only fine until you reach a savings amount above JSA limit), amounts to operating as a sole trader, as a business. It's a business context, a home, personal business. You don't have to be registered as a company.
The real answer is that you have to declare this to the JSA office in a form they give you, or maybe they would take the figures over the phone if you ask them, or in person. The benefits agency would then make a decision on the JSA benefit. It's up to them.
So, it all depends on when it happens that you stop selling your old stuff, and are significantly buying stuff to sell, paid for from the income of stuff you've bought to sell. It all changes when you're not selling your own stuff anymore, when you're not just converting your 'savings items' legally to 'savings cash' within the JSA eligibility limit. This is, of course a pretty obvious change which is hard to mistake.
You may get the JSA allowance lessened, depending on your circumstances, and you may have it removed altogether. If your income from selling items is irregular - say an average of around £90 per week, but some weeks £30, and some months only £200, you're probably eligible for Income Support. Then you basically receive that benefit and can keep £20 per week more than the benefit from a business style selling practise. After £20, you have to declare (like with JSA), and the benefit office tells you what you get and your eligibility.