I certainly don't do this for the money!!
The difference between £30 and £40 may be only a tenner, but pig producers operate on exceptionally tight margins. This is especially the case with rare breed or pedigree breeders. A hobby farmer or a business operating pigs as a sideline producing maybe one litter a year can afford to sell at that price, but an extra £10 for a farmer selling maybe 40 weaners a year from five or six litters is £400 (obviously!) and for many crofters and smallholders that's the difference between loss and break-even (we can only dream of profit). We sell fatteners for £45 and registered pedigree stock for £65.
If this is a source you trust, I'd be straight in there because it's a great price - £60 instead of £80 for a pair gives you £20 left over for about three bags of feed which will see you comfortably through the first month of pig-keeping.
I only advised caution after an advert in the paper here in Orkney, offering weaners for £15. Curious, I called and they couldn't even tell me what breed they were. I have developed a 'too good to be true' alarm system!