When valuing and considering to buy an online company - In general, do buyers think in-house software is a plus or minus?
Really does depend on company design.
For example, take a mail system.
If you run an online company teaching people how to setup their own MTA to send mail... verses...
An online company which allows sending email through either an SMTP relay or API call endpoint...
These 2x companies do roughly the same.
1st company is worth near $0.
2nd company could be work $1,000,000s/month.
Tip: Hire a bunch of consultants for 1x hour each, providing them details about your company, asking for how they'd design the company for highest resale value.
Then mashup/combine/merge the best of all the ideas into your business design.
Tip: Always design a business backwards, starting with your Exit Strategy + all will be well.