Compare like with like
Match card number, language, printing and condition, not just name and set. A graded copy is not equivalent to a raw card. For a lot, make sure the reference covers the entire quantity offered.
An example
An offer costs €90 plus €5 shipping. Compared with a €100 reference, the total is €5 or 5% lower. This says nothing about authenticity, seller risk or what you could receive on resale.
Choosing a reference value
Prefer recent comparable sales of the same version. Asking prices, trend estimates and converted foreign prices may differ. If suitable evidence is missing, keep the valuation uncertain rather than inventing precision.