The existence of the forum is a bit of a red herring. AFAIK the same applies in the UK as the US when selling goods and that’s the only aspect here you need to examine.
If you’re using a third party for the end to end merch fulfillment it’s not even a sale of goods issue, they take care of it their side including sales/import tax. The portion the third party pays you is taxed as income. The costs to run the Discourse instance and any other infrastructure is handled like any other expense. Your focus might be the community which generates the sales leads, but I can assure you that the IRS/Inland Revenue couldn’t care one bit.
If you’re selling merch direct then yes, that gets way more complicated, and also not a Discourse question. Your tax accountant should be able to answer all of this.