It really shouldn't matter so long as you don't buy some bottom of the barrel model and make sure the BIOS is fully up to date. Plenty of people still running perfectly fine using Z370 with both 8th and 9th Gen parts, with no problems.
Honestly though, at this point, it SHOULD be just as easy to find a Z390 board for within the same neighborhood as a Z370 board since both have been out of production for a number of years. I'd go with whichever you can find a decent model on, for a price you can live with, but most importantly, from a seller with a high number of sales and a very high satisfaction feedback rating. I wouldn't buy anything from any seller with less than 1000 sales or a rating of less than say around 96% at minimum if buying from Ebay.
If buying used through Amazon, make sure there is some kind of warranty at least.