The matter of materials certificates is only really for steel boilers built under and not before, the Pressure Equipment Regulations, but what does count is the completed boiler's hydraulic and steam-accumulation test certificates. Provided these do exist they do not need still to be valid if you are prepared for the possibility of some fault having developed since, that would mean a re-test failing. That could be why the engine is up for sale!
There is no technical or legal reason under the MELG scheme which most model-engineers follow, for not buying an uncertificated boiler… BUT…
It would be a gamble because it would need to be inspected and tested as if "new", possibly with all lagging removed (at the tester's discretion). … AND
Testing the thing at all would very much be at the inspector's discretion, especially if it is to an unpublished design. He or she might also decline a boiler of a type personally unfamiliar, although I do not see that a valid reason because the test is of a pressure-vessel, not a tester's knowledge of umpteen different designs.
At the very least I would request witnessing a basic hydraulic test to working-pressure only, to ascertain no structural leaks, to guide buying it. That could use the loco's own hand-pump, if fitted. It will also show minor, curable leaks within the fittings.
As a first step, examine the complete engine carefully: does it look well-built and looked-after, even if well-used, or is the overall workmanship clearly poor? You won't be able to see much of the boiler itself, but if the surrounding metalwork looks decent the boiler could be, too.
However…
Even that can be problematical. I was once personally involved in telling an unfortunate builder his new boiler, apparently to an LNER pattern, was scrap before we put a drop of water in it, despite his obvious craftsmanship. He was a retired coppersmith, though with no previous experience of miniature steam locomotives. He had faithfully followed the drawings he showed us, unaware of their very serious fault – almost no stays. The poor chap assured us he'd tested the boiler only to working pressure; but he had not noticed as we did, that it had collapsed the arched inner firebox. The poorly-printed drawings also bore no designer's and publisher's names, either, but he would not reveal their source….. We never saw him again.
On the other hand, if the loco appears generally good but at low price you might consider being prepared to re-boiler it if necessary; either building the boiler yourself or buying it from one of the professionals advertising in ME and MEW.
First step – find someone whose advice you can trust, to go along and examine the goods with you! If the dealer is reputable he should not object, provided of course you don't give him the impression of you thinking him guilty unless proven innocent!