What are Appropriate Terms for Selling Goods and/or Services? 
There are a variety of sets of terms and conditions and forms of agreement which you can find in Business Documents that are designed for sale of goods and/or services generally.
These documents are intended for use by sellers generally rather than specifically for a seller which is a charity, but a charity can make use of these as appropriate when selling goods or services provided that it makes changes to the document in each case as necessary to suit its particular needs and circumstances.
Each of those document templates is designed to be fair as between buyer and seller, but it must be stressed that they are primarily intended for use by a seller rather than a buyer in that they contain some terms that tend to favour the seller. A charity might readily decide to use such a template when acting as seller but it should, before using such a template for purchase of goods or services, review, amend, or add to the particular document as necessary to protect or improve its legal position (as buyer) as necessary. If in doubt as to whether to use one of these documents when purchasing or whether and how to amend it, you should take independent professional legal advice.
As mentioned above, when selling goods and/or services, a charity might decide to use one of the templates to be found within Business Documents. The following are a few examples of documents that are more likely to be of relevance to a charity for this purpose than most of the other general business sales templates in Business Documents.
Examples:
Standard Service Terms and Conditions with Deliverables (B2B)
Terms and Conditions for Supply of Goods and Services (B2B)
Standard Terms Conditions for the Sale of Goods (B2B)
Short Form Single A4 Page Terms and Conditions
Standard Service Terms of Business (B2B)
Charities might find others in Business Documents that are also relevant to their needs, and for information on selling goods or services, see Selling.