Objective
Can you argue that you are not selfish?
Yes, of course you can make the argument. It doesn't mean that it will be sound or definitive in it's assertions, but any point can be argued.
The argument itself is highly subjective - we cannot assert the validity of either side of the debate because there is no authoritative resource which to test the assertion against. One could just as easily argue that they volunteer at a homeless shelter to the benefit of their own ego or public image as could be asserted that their contributions are committed solely to the benefit of others.
No opinion can be well-maintained regarding the "degree" of selflessness an ("other regarding") action is motivated by without investing faith into a philosophical model regarding humanity.
You must ask: is a person's "selfish/selflessness" defined by the average degree of "selflessness" of their actions or is the quality entirely an attribute of their personality or "self"? How does one judge the degree of "selflessness" of an action - is it defined by the actor's motives, or the action's utilitarian consequences?
Then there arises the question of whether or not and arbitrary rating of "selflessness" has any definitive value: if one donates a large sum of money to a sound humanitarian organization that uses it to feed hundreds of starving children, and did so simply for a tax write-off and a boost of public image - does it really matter whether or not why they donated the money?
Subjective
I'm prone to think the motives are inconsequential - the starving children certainly don't care why they received sustenance so much as they care about receiving it. I would argue that regardless of the donator's motives, their actions ultimately produced a measure of "good" in servicing the individual humanities of others; I might so far as to argue that this common interpretation of "good" is the very reason the donator's actions may be viewed upon fondly by others. Perhaps they did so with "selfish" interests, but here the quality of their motives has no meaningful value.
Furthermore I think the nature of humanity and morality in general exists almost exclusively in "grey area" - things are very rarely black or white. Given our limited understanding of the mind, it seems reasonable to me that every action is motivated by hundreds of factors, both conscious and sub-conscious. I feel it is thus likely that all (non-effectively-self-regarding) actions exhibit and are driven by both selfless as well as selfish motives.
Can you argue that you are not selfish?
No, you can't make a sound argument that you or anyone else is either "selfish" or "selfless". Everyone exhibits both qualities simultaneously. I can only discuss and interpret the qualities with respect to individual aspects of their actions - and perhaps patterns that emerge over the course of many actions. But I can neither assert that I comprehend all of their motivations nor can I assign arbitrary value to the "degree of selflessness", and so it seems unreasonable to ever hope to objectively classify a person, pattern, or singular action as either or.