We were ready to go down that route but the builders we selected had a Howdens account so we use theirs. They showed us the quote from Howdens with "trade" and "retail" values side by side. Retail was mainly 2 * Trade. They just billed us the Trade amount.
Cheers
Dave R
MRMDF would be a better option than chip, since you can finish it to a good standard without needing to lip or veneer it.
Neighbour who is ex-MOT tester and garage owner still fixes cars in his retirement. He has an account with the local car parts supplier and all the invoices that I see lying around his garage show two columns, trade and 'retail' (or SRP these days). The latter is typically 80% up on the trade price.
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