If you're talking about selling names here on NamePros then I would say it definitely matters. If it's a higher value name (Something selling for at least $XXX on here) then they probably won't mind just having you unlock it and give them the transfer code so they can move it to their preferred registrar. However, if the domain is less than 60 days old then it can't be transferred because of the standard 60 day transfer lock on new domain registrations. On top of that, if we're talking about a $20 or $40 name it might not be something the buyer wants to turn around and have to invest more money into just to do a transfer of the name.
Domains that sell on places like Afternic will most likely want to do a transfer anyways (if it's possible), so the registrar when selling someplace like that might not matter as much, especially if the person really wants the domain. However, here on NamePros I would say the registrar does matter, especially if we're talking about cheaper names. Get them to someplace people actually use and have easy account creation/push options. GoDaddy, Dynadot, NameSilo, NameCheap, Name.com, and many others are used by lots of people on the forums, and they DON'T require you to buy something just to make an account. How many people do you see selling domains registered with IONOS on here? I bought a cheap domain at IONOS one time and wasn't a big fan of them. In fact, I still have never registered another domain there.. I would say the most important factors you need to be considering is:
1) Who is my market audience? Are you trying to sell names on forums like NamePros or on marketplaces like Afternic...
2) What is my price range? Are you selling domains for low-mid $XX or high $XX to low $XXX...