My experience of Gumtree has been excellent, although I've only sold through it. Like anything, ask for paperwork and receipts, if you're buying.
If it's an engine, then you definately want to see it running from cold, ask straight-forward questions about history and run a compression test (you can do this with a Draper £20 tool) without mechanical experience.
On purchase, get it serviced or service it yourself, renew starter rope and impeller as a matter of course, new plugs, gear oil and check electrical connections.
Nice find by the way. The 25hp is the same block, so would be my preferred choice.
Is that with or without VAT?