d/Selling Costs - ebay fees, paypal fees, postage.
In my experience, this is the part that everyone fails to take into account.
For starters, every item sold at £15 = £1.50 to eBay (=£13.50 left).
I've no idea what the material costs or the running costs of the printer are, or what (if any) fettling and other work is required; but even if you were to say it was 15 minutes per set of 4 pieces, at £10/hr (which is way undervalued IMO) that's £2.50 in time, so £11.00 left. Let's guess at £1 for material and electricity costs, so £10 left. We'll assume he's got the printer set up in his kitchen, so will waive any property costs.
Postage is built into the price, and judging purely by the pictures, I don't think he's going to get that into an envelope; so the small parcel rate will be charged. That's £2.85; let's allow him 15p for packing materials (£3) and assume he lives within easy walking distance of a post box (15 min round trip @ £10/hr is a rather convenient £2.50), so £5.50 on P&P
That leaves just £4.50 "profit", of which HMRC is going to stiff you for around 20% of that in tax. I'm assuming the guy isn't VAT registered (if he is, take £2.50 off the price before we even get started), so another 90p gone, so the final profit (excluding anything I might have forgotten, and excluding any consideration for the time it took to program the printer, feed it the filament, etc.) is just £3.60 per set.
Obviously, economies of scale could increase that profit (I'm assuming he has to make the trip to the post box for every item sold; of course, if he sold 100 of them, then the cost of getting to the post box reduces to 2.5p per unit instead of £2.50), but since he's only sold 2... I reckon he's probably going in too cheap at £3.60 profit.
Costs are a bitch, and outside of running a business, almost no-one seems to understand them. Yet they rack up pretty sharpish, especially on low volume stuff.