When valuing and considering to buy an online company - In general, do buyers think in-house software is a plus or minus?
The answer is it depends.
It depends on what you need the software for and how it adds value and creates a moat against your competitors.
I'm not sure what sector you are thinking of, but if you are an online retailer then probably not as the software in that sector is fairly advanced and mature, and solutions generally customizable.
That is to say up to a certain stage, once you become Amazon... you're best to have your own proprietary software.
Also, there is no point in investing in reinventing the wheel for day-to-day "commoditized" software unless doing so can unify and improve processes somehow, creating unique know-how or way of doing things allowing you to get an advantage over your competitors.
On the other hand, if you are a digitally native company focusing on disruption then you will most certainly need to create your own software for three reasons, firstly the uniqueness and level of customization, secondly the pace of development required, and thirdly an important part of company valuation when it comes to fundraising is the underlying company technology and IP which forms a valuable asset.
In terms of software development, it is not uncommon for startups to use 3rd parties to develop the initial MVP and once proved - and with access to funding - to then take it in house. It would be unusual for a growth stage/series A company not to own its IP and develop their software in-house