An <a class="localLink" href="http://schema.org/OfferForPurchase">OfferForPurchase</a> in Schema.org represents an <a class="localLink" href="http://schema.org/Offer">Offer</a> to sell something, i.e. an <a class="localLink" href="http://schema.org/Offer">Offer</a> whose <a class="localLink" href="http://schema.org/businessFunction">businessFunction</a> is <a href="http://purl.org/goodrelations/v1#Sell.">sell</a>. See <a href="https://en.wikipedia.org/wiki/GoodRelations">Good Relations</a> for background on the underlying concepts.