In general I'd say Amazon. Easier to buy from and easier to do the rare return to. Also their shipment tracking works a lot better than Ebay. Set up and use their text and email alerts.
I will use Ebay for outdated or older items which can apply to out of date RV parts.
You also have to watch two things on Ebay pricing.
#1, you'll find a decent or low pricing, but then watch out for the shipping costs. Often a low priced vendor will make up for the lower price in higher shipping costs so that overall, they're not lowest. I recently bought a couple of brake master cylinders from Ebay. One guy wanted $36 to ship one when it would fit in a USPS Priority box for less than $7. I asked him why the high shipping cost and he said Ebay set the shipping cost, not him. I contacted Ebay and they said that no, they didn't set the shipping cost.
#2, a lot of people assume Ebay has to be lowest when it's not. Another recent example was I wanted a couple more Swiss Army knives. The starting asking price, before people starting bidding the cost up, was more on Ebay than just buying them from Amazon.
So Ebay has it's place, but 95% of my purchases go to Amazon.
Bill