There are several reasons why this may happen (it happens to us all) -
You purchase items from wherever (UK) or Overseas supplier - you pay a Trade Price for the item (Perhaps in £Sterling or, $US) - there are lots of fluctuations with regards to the price whoever you buy from. Most prices have increased recently - many are blaming this on Brexit - in fact, I am fully aware that many of the Chinese Suppliers are looking at increasing their prices by 30% at least.
Assuming you purchase from a supplier in the UK, you are paying them for a product that they have purchased and imported - they want to make a profit and so the price to you is more than it costs them. They experience price increase - they will be passed to you.
If you are not VAT registered, and noting your PPI pricing, I would suggest you are not, you will also pay VAT on your items (if not VAT rated at zero) - dice, I doubt is Zero Vat.
If you buy direct from China, you will also have to purchase in volumes that justify the shipping charges - assuming you do this, you would then incur VAT / Import Duties - these are applied to the entire cost (not just the product cost).
No matter what of the two options above you choose, the Chinese Seller has the items on their doorstep - the cost to them is therefore likely to be a lot less. Add to this the fact that the Chinese Government subsidise sales, you are never going to get close to their prices.
Note, I have not mentioned the cost of the mail - the fact that they are able to get the items so cheap - they can factor postal charges into their sale price.
There are also several Chinese Sellers who use Fulfilment Centres in the UK - they despatch their items in Bulk Quantities and they are despatched by people in the UK who charge a fee to the Chinese. Many of those sellers have not paid duties - they are outside the UK and when challenged by HMRC they vanish - they then set-up new sales accounts and start the process again - each time, they avoid duties and taxes.
Fair - NO but, we are powerless to deal with the situation.
Unfortunately, the on-line sites that permit these people to sell are not concerned with those sellers - why should they be? You sell an item, the on-line site gets paid (a rather nice percentage) - they claim to pay the duties that they should and therefore suggest it is not their problem to pursue others.
Hopefully, this has shed some light on your concerns. I think we are all in agreement that something needs to be done to counter the situation - are you or I in a position to do something about it? Unfortunately not.
Perhaps, if those sellers that make it unviable for us all to trade make us all leave, perhaps then the on-line sites will look to challenge the matter.