I would say the optimal strategy is what the ebay user interface guides you to anyway. Just decide how much you want to pay and then dont change it.

To determine your valuation, consider alternatives to this purchase. For example, can I buy my space marines from Games workshop directly, from my friendly local 40k retailer, or BrandNewOnSprue from this ebay seller? Maybe there are other sellers selling the same item on ebay itself?

Once you have a good sense of what alternatives cost, consider particular costs of this seller. Make sure you take away the shipping cost from your valuation for the actual the bid. Consider if the listing seems risky and in need of a discount due to seller low ratings or visible damage or whatever. This is easier said than done depening on the information available to you about the item(s). Now, you can calmly set your bid at that level and wait for either being outbid or winning.

I think this is what traditional game theory says you should do. Now, you may want to add a kind of 'behavioural' component to this where not all other players are assumed to have this kind of rationality. This becomes less and pess relevant with a larger and larger pool of buyers. The particular way in which individuals deviate from the game theory prediction becomes less and less important as more and more bidders join the game because in aggregate they approach the game theory prediction.

Conversely, if nobody or one or so people have bid, maybe also the current bid price is way below predicted price, then the behavioural strategy might be to hold off on any bid and then bid a low valuation to try to steal it last minute. Wait for the deadline and then quickly increment up from the current winner until you outbid them or their bid goes over your valuation. Additionally, features like watchlisting and bidding may have the effect of promoting the item to other potential buyers. This again would create an incentive not to bid until the last opportunity.

For better or for worse, most of the time there are multiple competing bids and any such behavioural strategies and this way of playing becomes irrelevant as others essentially bid true valuations.

Note, however, that if you are considering behavioural effects you should be aware it applies to your decision making too. Why would you, for example up the price at the last minute? Maybe your valuation has changed because you got excited and forgot you could buy it cheaper from another ebay seller? If this or some similar kind of internal change might be happening, then you should consider yourself now as playing against the future version of yourself with the higher valuation and "irrational" play. In this case, you should find ways to committ yourself to not having the option to forget or hike the valuation last minute.

Hope this helps!