When buying a dovetail saw you need to ensure that it has good, resharpenable steel and has a strong back. Like tenon saws, these can be brass or steel-backed, the main difference is that the steel-backed saws are lighter. You also want to make sure it has 15 PPI or more (slightly smaller teeth) and has a comfortable wooden handle. Paul likes the turned (inline) handle for cutting dovetails, as found on the gent’s saw.
The 10” 15-PPI Spear and Jackson dovetail saw (often labeled as a tenon saw) with the rosewood handle is a good value new option, although these might be harder to find secondhand they do sometimes come up on eBay. Another name to look out for secondhand is Henry Disston which also has a comfortable handle. Paul also recommends this Crown gents saw which is available is the US, a 10" with 17 TPI.
Like with all saws, if you are not yet confident with sharpening them and you want one ready to go, Paul recommends buying a new one. Sometimes the new dovetail saws arrive over set, which will need correcting and some may also need further sharpening.
*Prices correct as of January 2020