So I've just had some PC's want to run a small trade business, buying foodstuffs etc in the Core worlds and selling them in the Outer Worlds. Are there any guides that cover this (either 3rd party or in one of the source books I've not yet encountered perhaps)?

I'm particularly interested in a list of products, trade locations and prices. I've started making a list of goods that each planet mainly imports /exports but the books sometimes contradict each other and there is no mention of prices or rarity.

Sure I can make this up on the fly but it would be good to follow rules if there are any.

Well, the corebook have rules for selling and trading in bulk. Also, if I'm correct, the colonist book has rules for background business.

You can find useful lists in WEG's Tramp Freighters and GURPS Traveller Far Trader.

Someone made a Google spreadsheet of trade goods across several categories. I've modified mine so much I'd prefer to link you to the original -- funnily enough, in my search I instead came across this fan-made rules supplement which you may find very useful. (I'll keep hunting for the spreadsheet.)

Edit: Aha! Here's the spreadsheet.