Hello, I am Keith, one of the experts on Just Answer, and pleased to be able to help you with your question.
The most important thing to remember when operating a VAT margin scheme is the maintenance of records. In particular the stockbook. Here is the advice from the relevant Gov UK web site:
You must record certain information for each item you buy and sell that you want to use a margin scheme for.
Stock number in numerical sequence-
Date of purchase Date of sale
Purchase invoice number (unless you made out the purchase invoice yourself)Sales invoice number
Purchase price Selling price, or method of disposal
Name of seller Name of buyer
Description of the item-
-Margin on sale (sales price less purchase price)
-VAT due (16.67% or one-sixth)
You must keep VAT records for 6 years. You have to keep records until you sell the item for any stock you bought more than 6 years ago that you plan to sell under the margin scheme.'
You proposal not to pay the artist until you have received the sales income is perfectly proper. Thus the 'acceptable time' in your question is not relevant.
I do hope that you have found my reply of assistance.