Just thoughts ... if it has a leak you have three options: fix it yourself, not too difficult if you have parts ... you need about 6 different sizes of O ring. I can supply my notes if you wish. They include O ring sizes.
Second option is to pay for it to be resealed... no real idea of a charge for this maybe £50?
Third option is to sell as is with price reflecting the reseal charge / hassle. I have no real idea of a price in a leaky condition but am sure someone else will. Maybe £100? but could be wrong. A risk is, of course, that the fault will not be cured by a simple reseal.
Sorry about this but if I do not know what a fault is caused by then I look for a lower price to cover the increased risk.
Cheers, Phil