Ifor 510s are well regarded, even more so than the more recent 511s which had some teething issues with mouldy plastic bits, so some people actively prefer the 510 and look for them.
I would say any buyer of a trailer should be making their own inspection or better bring a qualified mechanic to do so at the time they are buying, since any certificate altho a bonus would be out of date the day after it was done.
good luck on your friends sale.